I agree that having numerical proof that someone is worthless does not make them worthless. This isn't Heisenberg's FFXI. It is entirely possible to measure something without affecting it.

That said, if they're worthless with or without the parser, why not get numerical proof?

Some of the better linkshells I've done events with use multiple people parsing during profitable events (like Dynamis) and give bonuses to the top healer and DD, then arbitrarily pick an MVP (often a support job or something like a puller). It isn't as wasteful as you'd think, and people don't afk nearly as much.