T30s are useful, IMO!
So, I am going to start the opposite side of this discussion and say that I think T30s have their place even as a monthly exercise. I have been using them with select swimmers ever since they were given to me in a meaningful way. Granted I was told why I would be doing T30s and they were not given as a mindless and useless exercise. I agree that they should not be given universally to all swimmers without regard to form or ability level or prior training, but if we are talking about athletes being coached by TI coaches or athletes that are self-taught TI swimmers, the T30 does some good things in the right context even without a Tempo Trainer (and I might even argue that they SHOULD be done without a TT). It just needs to be used as a test and not as a training set to make you faster...I agree it won't likely do that.
Here is my reasoning:
If you have coached athletes for a while or even if you just race yourself, you probably know how important mental confidence is to an athlete's performance. A well-trained athlete who doesn't have the confidence that they can make the distance in the time they are shooting for even if they physically can probably will not accomplish their time goal. The mind is very powerful and I am sure TI coaches would be the first to agree with that. I mention this because for a lot of triathletes (which is a good portion of the TI swimmers and coaches), a 30 minute continuous swim is something that they will likely have to accomplish in a race depending on what race they are doing and is at least a closer simulation of it than a 200yd repeat test (which also have their place...not discounting those at all here). Having a monthly test to see how fast they can accomplish that long of a swim is not a bad thing. It builds confidence as long as they improve each month and that is extremely powerful...and I think, for most triathletes, an improvement in a T30 is more powerful for their confidence than an improvement in a 200yd repeat as a monthly indicator. The T30 is a test, not a training tool...you are not using it to make them faster...you are using it to see how the other work (stroke length, stroke length work and such) has made them better.
In my own swimming last year, I did a T30 in the first month of training. I swam 1875 yards. I then worked for 6 weeks under the coaching of another TI coach who shall remain nameless to protect the innocent, on various technical aspects of my swimming...no mindless swimming of laps...very technical work in some cases and some speed work and a whole slew of things including tempo trainer work. After that 6 weeks, I redid my T30. This time, 2075 yds. A 200yd improvement in 6 weeks at the same effort level. This did huge things for my confidence in the program. Sure, my improvements at shorter yardage did good things for my confidence and showed that the program was working but nothing affected my confidence in a positive way more than those T30 improvements. I was training for a half ironman at the time and was aiming to go under 30 minutes by a bit. I was almost there in the pool and knew I would be a bit faster in open water. I knew what I was doing different because the other training during the six weeks had wired my brain to swim with a different tempo and length in order to get those extra yards without extra effort. I could have told you at almost any point what tempo I was swimming even though I didn't have the TT on.
I suppose you could argue that the athlete shouldn't need this confidence booster from a long swim and should be able to accept that the improvements in the shorter swims are proof enough that the swimming is improving and he will be able to complete his race. Good luck convincing the average type A triathlete of that. They will go out and do the longer swims as secret workouts just to make sure. Or, you could tell him that he needs to save those longer swims for open water tests, but what about the athlete that is doing IM CDA in Idaho in June and lives in a cold area of the country and can't open water swim in the lakes until May? How does he gain the confidence that he will be able to complete a continuous 2.4 miles at his goal pace?
Now, why do I say that I would even argue that you should do the T30 without a TT? Well, I think one of the things that you are trying to do with your training is teach your brain to memorize what it feels like to swim at certain tempos and stroke lengths. It seems to me that your T30 test each month should also be testing how well you can maintain your goal effort level, tempo, and stroke length without it being dictated by the tempo trainer. I think it would be good to count strokes on a length every few laps just as a check to see if you have added a bunch of strokes so you can refocus if you have but I don't think you need the TT. You are likely not going to race with the TT (although you certainly can in USAT races), so why test with it on? You need to be able to test and see if you can make your brain hold the feeling of that perfect tempo and length for an extended period without the TT in my opinion.
So, in general, I think if your training sets that are intended to wire your brain to swim at different paces throughout the month use the TI algorithm and you work consistently on stroke rate and stroke length and mindful swimming and making improvements to your stroke, then you can certainly use the T30 as a test to see how well the changes have taken hold for longer swims that are closer to your distance racing.
One more thing I will say though as a caveat as I alluded to at the beginning. This only really applies to swimmers that are ready to handle T30s (i.e. they are at least close both in fitness level and stroke mechanics to swim 30 minutes straight without gross drops in pace and form). For instance, I have swimmers training for sprint triathlons that currently have a hard time making more than 5 laps in a row without having to stop. They have stroke issues and fitness level issues that would make a T30 useless. I have other swimmers training for longer races or just with more experience that can come close to maintaining form and speed over 30 minutes without a problem...I think a T30 is an appropriate tool for them. During the rest of the month we work at shorter distances, we work with the tempo trainer, we work with swim golf, we work with some sprints or even "cheetah sets", and then we might test with a T30.
Just my thoughts. Hopefully I haven't misunderstood your intent, Terry, with this thread.