It still causes strategy, particularly because the user must remember that Perish Song affects him as well. It's handy to force a switch on the opponent (of course you are already planning a swap during the countdown), and if he doesn't swap, you witness the collapse of a giant, hypothetically speaking.
I ran into Perish Song for the first time in Black yesterday, with the opponent Jynx using it immediately; I found myself knocked out before the Perish Song countdown took effect (hence I never got to swap), but when her countdown on the Jynx in question reached one, she immediately swapped it; in fact, the Jynx used it on my new entrant, so I immediately swapped at the same time before losing a turn. Eventually the Jynx was left by itself and Perish Song would have effectively caused the loss, so it had to lose the normal way.
It's a good way to impart some strategy into the battle. I have no issue with it being left in, but sparingly. One boss battle is definitely good as it introduces the move at least once in the game and also points out a Pokémon you can have learn the move if you ever capture it.
Cyclone