While it's all subjective, I did a quick Google search for top reasons to JB your phone. Nearly all the hits that came back were from smaller, lesser know blogs/forums/etc and nothing from any major, reputable sources. As far as JB'ing goes, did find something from Pinch Media who stated their analytics showed that, roughly, 10% of all iPhone users are jailbroken. This was as of 11/09 I believe. Personally I think the percentage is a little higher but who knows.
Of all the "Top 3/5/10/15 Reasons to JB your phone" blogs/lists, nearly all of them had unlocking either towards the middle or bottom of said lists. And in these polls, the top "reasons" to JB were for 3rd party apps and/or personal modifications.
Personally, I only care about an unlock for resale value of my phone. I JB'd specifically for the themes/customization. The 3rd party apps were a blessing in disguise and now I couldn't live without them.
Again it's all subjective since none of the sources I found were reputable but it gives some insight to the main "reasons" for JB'ing your phone, if only on a somewhat limited scale.