I think it's at least as "pretty darn good" as the other rules that allow for a player not being able to be sure or "know." Why is this one worse? If players abuse the rule, they aren't abiding by the spirt of the rules. If you include the ball not being on grass in your definition, that's pretty hard to abuse. If they aren't virtually certain (or whatever certainty you want to assign) then they're cheating. If we're going to ban cheaters from golf courses, at least I'll finally be able to get whatever tee time I want.
As to slow play, it has been pointed out many times in this thread how rare a ball ending up in a divot in the fairway is. That tracks with my experience. It seems to me that that alone would minimize the effect on pace of play. Further, a better lie makes it less likely a crappy golfer will hit a 30 yard worm-burner requiring two shots for what should have been one. That seems like improved pace of play. Any altercations would be no more than any other rule over which two competitors disagree as to application. I disagree that a divot relief rule would be any more frought with peril than the embedded ball rule or the way we determine where to take a drop from a lateral hazzard (red penalty area - whatever) from a long distance away. I also think its a weak justification to not have a rule because people are going to fight over it.
I see and at least in part agree with your point about divot relief being counter to the principle of playing the course as you find it. That argument makes sense, though I think the embedded ball rule, for one, provides precedent for disregarding the principle when the powers that be so desire. That does not mean they should disregard it lightly though, which I think is (mostly) your point.
For the record, I don't actually mind the rules requiring us to hit out of divots. I'd rather not and lean towards the change, but I don't feel strongly about it. I'm OK with the tradition and the need to deal with adversity even after hitting a (usually) good, or at least well aimed, shot; I can see that it adds something to the game. Thems the breaks, as they say. But, I think many here overstate the the strength of the case against change.