Next person who says running a custom rom is about running a custom wallpaper is getting slapped. You can run any wallpaper you wish on any Android phone you wish easily, without even so much as root.
Custom roms allow full customization of the OS, what apps are installed, where they're installed, and how well they run. CM, for example, includes a fully redone music app that is light years ahead of stock Android. Most first class roms also fully zipalign and deodex every apk, making for a much more fluid UI, something you don't get in Stock. Furthermore, CM includes a full set of ARM Neon optimizations, think SSEx instructions for ARM processors. The Droid 1 with CM 5.0.8 and now 6.0 RC1 absolutely flies, there is no lag or stuttering what so ever. With a 1Ghz OC, the infamous Android menu stutter is nearly completely gone. Speaking of over clocking, because that 1Ghz OC is done on a low voltage kernel, my battery life nearly doubles over stock. OS upgrades? Without root and custom roms, you're stuck with what ever upgrades the manufacturer chooses to give you. Droid 1 owners were enjoying highly polished 2.1 based roms months before the official update, just as we're enjoying 2.2 based roms with barely a peep of an official release from Moto/VZW. The same will hold true for Gingerbread.
Those of you contemplating the Samsung Galaxy S phones, rooting and custom roms could very well be the only way you're going to to get Gingerbread, if not Froyo.
When you buy a piece of hardware, it should be your choice on what you wish to do with it. We can sit and discuss the pros and cons to rooting and custom roms all day long, but ultimately, its a choice thing. If you don't want to root, you're not forced to and you can continue with stock and whatever upgrades that the manufacturer pushes out. For others, we demand the ability to do without devices what we please. We understand the risks fully.