I haven't rooted my transformer, and I don't really see the point in doing so (yet).
I know you asked for people why they rooted their Transformer, but as someone who typically roots their devices within a week of a "stable" method being found, I wanted to chime in on why I haven't.
For me, I have three main reasons I typically root my devices.
1) Cyanogen Mod: I personally prefer a "stock" phone, but I love the tweaks that Cyanogen built into their rom. I toy with different roms from time to time, but no matter what the device, I always find myself coming back to the blue Android. Since the source for Honeycomb will not be released, there won't be a version of CM Honey (Honeyogen?) or any realy custom roms.
2) Up to date software. My first experience rooting was with my OG droid when 2.1 was released AOSP but not officially. Security issues aside, I like having up to date software because I can try out all the "cool" bits before everyone else. Asus is actually doing an admirable job with this. According to thisismynext, the Transformer should get 3.1 starting next week, and I'm ok with them taking 2 weeks to get the update pushed out considering my MicroSD slot works.
3) SetCPU. I'm not a speed addict. I actually haven't overclocked a device since my OG droid, but I love this app because it allows me to underclock my devices. This means that it consumes a LOT less battery life with the screen off. My Transformer typically lasts me 2 days, so this isn't an issue with me yet.
My main reasons to root a device aren't relevant (yet) and there are no compelling "tablet root" apps (yet) to tempt me. Asus has done a great job of tweaking some of the letdowns of Honeycomb without plastering a useless skin all over the place (letting it update quickly). So that's why I haven't rooted the device, but I'm also interested into why some people took the plunge.