Because on my samsung phone i use aosp roms to get on the latest android version and have a slicker OS but the kitkat roms dont have many features yet compared to the 4.3 ones so i use xposed to add what i need.
The nexus is on the latest and most stable android rom anyway so i dont need a custom rom. Xposed does what i need.
Also, because xposed is modular, i can pick and choose what features i do and dont need and each module apk is being updated individually so theres no need to flash a nightly/weekly just to get one improvement.
I honestly think using a clean stable or stock rom, custom kernel, and a framework is gona be the future of modding.
Another nice thing about xposed modules is that titanium backs them up with their settings so when you clean flash a new rom, its really easy to get all your customisations back