Yes, I guess if you can tolerate the lag of your phone along with the lag of your bluetooth gamepad. Really, it's quite terrible on games that require any sort of real precision. There's plenty of videos on youtube that shows that lag can be ridiculous--one video showing Sonic for the Genesis on a phone with 9 frames of lag vs 5 on an LCD monitor on a PC. Real hardware tends to have between 1 to 3 frames of input lag--whatever the underlying game engine has. There's been some recent efforts to combat this--Retroarch incorporate some runahead emulation--and if you have a really low latency LCD monitor it can be as good as a CRT (or in the case of the GBA its LCD)--or maybe even better with Retroarch? On the other hand, you have to have a system basically capable of running the game at 200% (or more).
Put simply, good emulation should only introduce at most an additional frame of input delay, and that's definitely acceptable for the vast majority of people (including me in almost all cases). But there's a lot of emulation solutions like Android+Bluetooth that are just terrible for anything interactive. I guess you could through a lot of practice compensate for this with memorization, in most games, but it's really incomparable to real hardware or decent hardware with a decent screen and a decent input device. IMHO, anyways.
