I have KEF R3 bookshelf speakers. They are substantial for bookshelf speakers. After owning them a few months I decided to add a sub. I added a KEF R400B, It definitely added another dimension to the sound. Although better, I still felt like I was missing something, so I added a second R400B. It has taken a while to get it right, but the 2 subs really complimented the R3s and allow them to do what they do best and the subs take care of the rest. I don't know about all applications, but adding subs worked for me.
Why I'm not adding a sub to my 2-way monitors for music
I've updated my blog post with more data, more analysis and the conclusion that in my particular case I do not need a subwoofer. I encourage everyone who is on the fence about adding a subwoofer or not to read and comment. I think that either way you'll be able to make more informed buying and configuration choices.
Everyone's situation is different but I hope the post helps you get to the right conclusions.