Unless the league is set up in such a way as to prevent people from picking up players and then dropping them, I think the strategy of picking up and dropping spot starters while punting ERA and WHIP in h2H leagues, and of grabbing players to prevent another team from getting them is perfectly legitimate. If the league or commish doesn't want that kind of activity, there are several ways they can counteract it by setting up the league differently pre-season.
First of all, the easiest way is to just allow only weekly waiver moves. That kills 90% of the constant roster turnover problem. Alternately, you can set transaction limits, innings maximums or requirements that a player stay on your roster for a certain amount of time after being picked up. If none of those restrictions are there, using loopholes to your strategic advantage is legit.