A loop against backspin comes at you differently than one against block or topspin. At the lower levels, the loop against backspin is often the only loop they see, but as players reach the intermediate level and beyond, more and more they face loops against just about any deep ball.
And yet most players practice mostly against loops off block or (at the higher levels) counterlooping. Much of the reason for this is how easy it is to do drills where one player loops, the other blocks. You can do a continuous drill in this way, facing a loop something like every second, and rapidly become proficient at blocking (or at higher levels, counterlooping) such loops.