As a THM, there are times when single target is a better choice.

1. Large trash pulls that are going down easy. Fire I uses far less Mp, and by the time Fire I I casts whatever I targeted may be down and I'll have to retarget and cast again. I can single shot what's still alive faster in some cases.

2. Same situation as above, if I feel having every mob come after me will end badly.

3. Small spaces where I'm likely to pull something the tank hasn't.

4. I may take the kill shot on something with Fire I if Mp is getting low before I transpose and resume aoe.

Those are the reasons aside from a crappy healer or a tank that can't keep aggro.