If you want to have control over the UI and functional aspects of your phone, it can be - the thing is, there are enough leeways currently - Rootless XPosed, Samsung NiceLock, custom DNS for system-wide adblock, etc - that it's often not needed. Custom ROM scene pretty much dried up because of this. But with vendors more than ever locking up, and Google cutting back features and API access agressively, this could change.