You could say the same about praise or play as treats, they are obeying the command because they want the praise or to play.

It doesn't matter what you use to reward them (treats, praise or play) so long as the dog see's it as a reward that's all that matters.

Some dogs are more responsive to a particular reward but that's not to say you can't use a different one, it might just take more effort to train them.

We train ours using a combination of all the rewards. It's good if you can do it just for praise or as well as both or one of the others because you can give them it anytime, anywhere. It's not always possible to have treats or a toy to hand!