Part of the issue is you don't want 100 features but only one person uses one feature, like word or Excel. If you do have a lot of features that are rarely used, you should hide as many as possibly behind an 'Advanced' area.

If the feature is useful to a large set of your users, then I would agree and add the feature. If it is only 1 user asking for it, then I would not add it. I tend to be like you and think, its only 1 line of code to make it work like they want, they are a paying customer, why not make them happy. But I've been burned by this. I had a customer who really wanted a few modifications. I made 1 that would help everyone using the service. I was going to start the second one against my better judgement, but right before I did, the person using the software quit her job (where she was using it), and her replacement didn't want to use it.