Even if they fully pay for their own costs, how can you possibly trust them?
These aren't like the undercover restaurant reviewers of print media circa 20 years ago, booking reservations in friend's names and showing up in disguise.
Anytime one of these bloggers shows up on a plane or at a hotel, that account has been flagged and they are shown the best face available by that travel provider.
All of that said, I was just at the Andaz Papagayo and it was a pretty nice place, and a good deal at 15K points. I'd take serious issue with Ms. Hull's review of the breakfast, but on the other posts I've skimmed, she's got a reasonable grasp of the reality of the place, even if hers is a bit shinier than mine would have been.