It is not only acceptable to form armies specifically around activity, it is even the suggested approach on the Wiki. The guideline is to not force players to comply to your strategy, but form your strategy around what your players do.
Players - not just the generals, also those involved in lower ranks - want the success of moving rapidly, surprising the enemy and winning not by numbers but by determination. It's totally fine with me, as long as nobody is forced to adapt his real life to a game. If people willingly do it, that's theirs to decide.
