You're right. Everybody's situation is different, and the type of sub makes a difference, too. In my small 2.1 setup I use a 300w RMS 8" sealed sub that has substantially improved the sound of every bookshelf speaker I have tried. It doesn't hurt that it has a rare high pass filter on the high level outputs.
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.