Yeah, that's my biggest gripe with RE6 mercs. It's not as bad as RE5's boss holding (try holding a reaper, or a chaiwsaw majni and you'll know what I mean), so I can somewhat tolerate it, but it's still really freaking annoying.
I actually don't have much of an issue with this. The only ones who will even attempt to hold the boss are mostly those that go for crazy high scores (1.3 mil and above), and that's certainly not needed for an S rank, which is all the majority of players will want.
Keeping a boss around actually adds a bit of pressure, and though I've yet to successfully hold one until 150, I'm gonna keep trying.