Actually, only the Datel Max Media Launcher can't enter "suspend" mode (sleep mode), and even that can be modded to work. Passcard 3 sleeps perfectly.
Also according to tests I've read somewhere, the battery loss with Passcard 3 compared to Flashme is miniscule, only about 15 minutes (this was tested using an M3, though)
Apparently you can't use Download Play if one of the DSes is booting up using a Pass device or Flashme, unless ALL the DSes that are downloading it are Flashed. Normal multiplayer (where everyone has the game) works fine though.