I really don't think it's wise to be blithe about this. This is a common and thorny design trap in all sorts of game design--the problem of having an action that has the risk of being either so overwhelmingly optimal there's no reason ever to not take it, or so pointless that there's never a reason to take it.
Like, if you just baldly implemented this "three actions" thing in 4e, then every single round, every character would always use Total Defense as one of their three actions, because there's really no reason to not do so. You're functionally giving yourself a free feat, every round, forever--unless you know you're just not going to get hit even slightly, in which case, again, it boils down to a trivial calculation, not an actually engaging decision.