Using Rel s5 Sho subs with my Thiel 3.7s. I would never go back to no subs. There is a presence and expansion of the sound stage that I would very much miss. That said, if you listen to chamber music or music without much lower frequency information, I would image you wouldn't miss having subs in your system.
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.