The problem with modern smartphones is that they run on battery, even when plugged into external power. You cannot use it without the batteries without some more hardware hacks. As the batteries aren't designed to be always in a fully charged state, if you decide to use the phone with the battery you have to carefully monitor the batteries too if you keep it powered 24x7. Old batteries can also bulge or, worst case, even explode. Another thing you need to consider is cooling - Smartphones are designed with the idea that their CPUs won't run at 100% always. A smartphones CPU, Camera, battery etc. tends to get hot on prolonged use, so if you are, for example, planning to use it as a mini webserver or as a 24x7 streaming cam, you need ensure it remains adequately cool. And ofcourse, the best smartphones for these kind of projects is one capable of running Linux.