They play a role in the game, yes. But that role isn't very significant. After all, this game can be beaten without a single Support rank gained. And as such, Supports are pretty much extras for the player.
The problem with this statement is that Athos exists for one chapter only. That's hardly enough time for anyone to make a contribution big enough to make a big enough difference to be counted as a top tier unit. Athos may be good, but his availability (or lack thereof) is a noticeable flaw on his part.
Sain/Fiora and Sain/Priscilla requires 40 turns per Support level. Not happening. Sain/Isadora requires 80 turns per Support level. Also not happening. Serra's grounded and her Support with Sain is as fast as Sain X Fiora or Priscilla. Louise is grounded, her Support with Sain is as fast as Sain/Isadora, and she's busy providing backup to Pent anyways. Kent's the most viable option he has for Supports from an efficient standpoint. And even then, 17 turns for a C, 27 turns each for a B and an A, totalling 71 turns for an A, isn't guaranteed to allow Sain and Kent to build Supports.