Random Matchup BANS pokesaved Pokemon
Very rarely do pokesaved Pokemon bypass the system, so don't be suprised if one of them is actually allowed, but most of the time it won't let you. The game has a system which detects if you "hacked" your Pokemon, and pokesave is considered hacking. This is usually because in black and white, it is literally impossible to get a 6IV Pokemon, unless they hack and use pokegen and all the people who use pokesav usually use 6IV ones. If you RNGed a Pokemon, or get a pokesav Pokemon with 5IVs (so like a dragonite that has perfect IVs except for special attack which its moveset wouldn't need) then it may work.
For tips on RNGing, check this smogon guide: http://www.smogon.com/ingame/rng/