There is virtually no "real" reason for me to root etc. these days:
- sure you can do some nice wee things i.e. modded youtube HD app, however, it isn't necessary especially when I can just use the browser or BS player to stream HD videos and then for most other things, there are other ways without having to root
- I don't like doing full backups as I always encountered some annoying niggle when I restored backups, rather start fresh and its not like I have loads of app anyway since sense works great for me, does everything with ease and without problems, also there is no need to root for titanium now as you can use helium backup app or by the looks of it it will be implemented within android soon...
- as tp02 pointed out, not all apps work with phones that are rooted and iirc some banking apps don't work either.
The only useful/main reason is for adaway since adblock has annoying niggles, however, I am sure that it will be updated to work better.
And plus I don't have touchwiz on my device so no need to root in order to freeze all that bloat

Besides anything I don't use/like is disabled or hidden via nova
As for custom ROMs, again the only/main reason for me, is for custom themes, with android phones these days, there is very little need as battery life, performance, stability etc. are already pretty much perfect.