Why shouldn't data be modified on an HTTP GET

Question

I know that using non-GET methods (POST, PUT, DELETE) to modify server data is The Right Way to do things. I can find multiple resources claiming that GET requests should not change resources on the server.

However, if a client were to come up to me today and say "I don't care what The Right Way to do things is, it's easier for us to use your API if we can just use call URLs and get some XML back - we don't want to have to build HTTP requests and POST/PUT XML," what business-conducive reasons could I give to convince them otherwise?

Are there caching implications? Security issues? I'm kind of looking for more than just "it doesn't make sense semantically" or "it makes things ambiguous."

Edit:

Thanks for the answers so far regarding prefetching. I'm not as concerned with prefetching since is mostly surrounding internal network API use and not visitable HTML pages that would have links that could be prefetched by a browser.

Answer 1

Good reasons to do it the right way...

They are industry standard, well documented, and easy to secure. While you fully support making life as easy as possible for the client you don't want to implement something that's easier in the short term, in preference to something that's not quite so easy for them but offers long term benefits.

One of my favourite quotes

Quick and Dirty... long after the Quick has departed the Dirty remains.

For you this one is a "A stitch in time saves nine" ;)