Personally, I've always thought that the idea the DM and not the player gets to pick the creatures is stupid. Why would I use a spell, a nice 3rd level no less, when I wasn't sure what the results would be? As a DM I've always been of the mind that the players already have a lot of limitations in their framework in how they interact with the world, and I don't, so why would I take away any reasonable choices that the rules leave them? I'm struggling to come up with a scenario where letting the players pick the creatures summoned would unbalance the game in any way I couldn't deal with as the DM, so I let them pick.