The point of WAR is to provide what value is "in a vacuum" void of all circumstances and situations. It's a value that is the same for all teams, no matter where they are, no matter how good they are.
And yes, anytime you replace a 0 WAR player for a positive WAR player, you are hurting the team. But that assumes that positive WAR players are easy and free to find, and that teams have an abundance of them. When the Reds were running out the Tim Aldeman's of the world the last few years, it was because they didn't have any other options, and weren't willing to pay to get a better player. It wasn't because they had a 2 WAR pitcher they were hiding, bu wanted to pitch Aldeman anyway.
There were over twice as many players in MLB last year with a 0 or negative WAR than with a positive WAR. That should tell you a lot.
And again, what "competitive" means to one club may not mean "competitive" to another. That is why WAR needs to be looked at in a vacuum.