With that "logic" you don't put a rookie, UDFA at that, in to defend on the goal line stand that will decide the Super Bowl.
Do you?
Apples and oranges.
Goal line stand requires multiple players. Chances are, due to salary cap ******ed depth, there may not be another choice or a lot of good choices (which may have been the case with Harper and the PR duties due to injury). Further, the goal line stand player being a rookie does not mean failure, it just may mean an increased chance at failure due to inexperience. Side note: In Butler's case there wasn't a lot of good choices. I doubt anyone was feeling mightily secure about Butler being the wall between a SB win or a SB loss (thankfully it worked as it did). Unfortunately for Arrington he was sucking it so bad we had to go with Butler.
For a PR catch, it is that singular player between a possession and a losing a possession. Over the years have you noticed how BB almost always puts the sure handed vet out there? BB has had some explosive newbys and even explosive vets that he could put out there yet he almost always went with the guy with a few years under his belt and proven sure hands.
Look, the team is missing a lot of parts. Harper was pressed into duty -- a duty he was not experienced at. IMHO the inexperience cost us. IMHO Harper of 2 years game experience probably makes a good play on that ball. Either way, whether he was pressed in to the role, shouldn't be doing it, is the second coming of Jules, it really doesn't matter. It is case closed on that play and that game. Harper learned from it, BB learned from it (and we all learned a late sunday evening hangover and a loss to Denver is a bad combo).
As far as Harper? I feel bad for the guy. This was a HUGE stage for an unheralded fringe player trying to make a career out of the NFL. He had a great catch last week erased. This week he had the chance to execute the important possession change and it got away from him (you could see him take his eyes off it). For a guy like Harper you don't get many chances to 'get it done', prove your worth, solidify an NFL career). So I personally hope the young man is brought back to the PS. The ability to bounce back from a stinging mistake is a lesson for all NFL players. Look, if he was good enough to make the PS and then be promoted, it is hard for me to believe one mistake means he isn't even PS worthy now (though BB and staff may have seen other stuff like route running and downfield blocking that was not good. That may have factored into the decision to release him). Unless there are pretty bad mitigating circumstances to Harper's play, this one play can't be the difference between being cut versus PS and 53 Roster can it?