I see a difference between 'niceness' and 'positive reinforcement'. The "everyone deserves an A for trying" approach is 'nice' but it generally isn't skillful positive reinforcement; I think a major problem with it is underestimating how much it rewards behaviors that look like trying but aren't trying.

There's also a basic value question- if you're trying to build self-esteem, it's not clear that an "A for trying" approach overvalues positive reinforcement, though if you're trying to build understanding, it clearly would be a misapplication of positive reinforcement.