My 2 cents
I use a little carrier, so my stock ROM leaves much to be desired. I decided to flash when I realized many ROMs include most of the tweaks preinstalled, saves some time and gives me some cool extra features.
Since I started cooking a ROM, it has just expanded. It is nice to be able to dictate exactly what pops up if you hard reset, and when tweaking its always a possibility that you tweak something the wrong way and have to hard reset, starting all over.
Another big reason I flash is WinMo 6.5 (aka Windows Phone 6.5 now apparently). Been using Titanium for a while now and don't know how I used TF3D. Flashed back to stock a few days ago on general principal, and boy does it suck comparatively
Though if my carrier ever releases a phone with 6.5 on it I might just snatch that bad boy up and dump it so I can tweak it and post a speedy sqeaky clean ROM.
Anyway, if stock works for you, more power to you! Less you have to worry about if you ever have a claim problem