I did a semester's worth of Scheme programming in college...it was alot of fun, but it's use seemed fairly limited. On the other hand...Scheme *is* used for writing scripts for the GIMP if i remember correctly...
Scheme is not limited at all. In fact, it is very powerful if you have the proper packages. That is one of the strengths of Scheme, it is simple yet very powerful. The only drawback I see that it has, is that the complier I used for it didn't actually compiled a full program. It compiled an executable that worked, but if you wanted to port it to another computer you had to copy all the source with it. It can be implemented for web dev; in fact, I forgot who, but Yahoo bought a Scheme store script from someone before for a lot of money. I highly recommend learning Scheme; it's not only fun to use but it's a very good learning programming language. Try looking for DrScheme in Google if you want a good compiler. Hope that helps!