You've done really well! I know a few people who still use nose clips, possibly because of allergies to pool chemicals, but I think it is more relaxing to swim without. Plus, if you ever swim in sea water, it's one way to keep that horrible taste out of your mouth (mostly).
My T.I. swim coach has just written a book called Effortless Exercise, and breathing through the nose is one of the things he emphasizes. Apparently there's a nerve running through the nose (vagus nerve?) which affects your heartbeat. He writes that babies breath through their mouths only when they are stressed out enough to start crying, so bypassing that nerve by breathing only through your mouth tells your heart/body that you are in a stressful state. Obviously you can't really inhale nasally when swimming, but breathing out through your nose would still stimulate that nerve.
I tried breathing only through my nose yesterday morning in a spin class and although it was hard to do, I was amazed at how fresh and energetic I felt all day afterwards, despite having my heart rate around 130 for most of it (I'm 50). From 131 to 140, I had to start exhaling through my mouth, from about 141 up mouth breathing only had to come in (just one of the intervals, fortunately) Normally that class makes me feel tired and drained for the rest of the day. I can't wait to try this with running.