Rules Clarification on "Agreements" - 2ndM

If you have a large force on a game such as StB or LMotH and there is a slightly smaller force and say 2 or little even smaller forces still left in the game contesting infinately and such is it legal to allow the slightly smaller team to tag and then walk off the flag allowing your team to win? The rules say "if there is no benefit to your team", but there is since you won't have to fight the slightly smaller team. An epic situation similar to this occured in OpOp, just looking for clarification on this before it happens in a game.