I'm someone that uses rain / rain + thunder / waves on seashore sounds to help sleep - it's quite noisy where I live and these sounds mask outside interference - but I can't see myself moving away from the mp3 player (Cowon J3) solution I use to this website. The main reason would be the mp3 player does just the one thing I want, i.e. playing the sounds, and this is very power efficient; I wouldn't want to leave my PC / tablet / smartphone running this webpage.
Also, an mp3 player allows me to loop or create a playlist, for example with 'heavy thunder' at the time of night when I need the most noise-masking, and with 'light rain' when I need the least.
Finally - and, yes, I realise I might sound a bit kooky here - I want to be able to select my own loops and samples. Some mp3 sources for rain I've found on the 'net have lots of aliasing noise, and I simply can't sleep as my mind focuses on the artifacts rather than just drifting away.
A couple of the above points (especially the 'timed program' one) might be able to be addressed by your implementation. And while I think I'll stick with my implementation for now this is because I've already done the 'hard work' of setting something up; for 'beginner' or occasional noise-maskers, your site is a quick and easy solution. My suggestion, then, would be to focus on this group as a target audience. Good luck. :^)