Arlien hit the nail on the head.
Most low and some mid rated teams don't even PvP casually. What you have, that pisses of both semi-casual mid-rated arena players and hardcore arena elitists alike, are the PvE teams that point farm. They just gather a group, put 10 games in per week and farm points until they get the weapon(s) they want, then disband, their sole purpose to collect weapons for use in PvE. A routine that consists of no more than a half hour of half-hearted play (excluding wait downtime) hardly constitutes effort. Arena's about a lot more than just getting your 10 games in and calling it a day.
To bleeding-heart PvPers, arena weapons and gear are more than just items. They're status symbols, and the feeling of accomplishment is significantly diminished when you have point farmers and scrubs running around as decked out as you are. It's really no different than the PvEers who buy their gold in bulk to grab every AH BoE epic they could find for their toon. This rating req on S3 weapons is primarily aimed at stopping this, if nothing else. Unchecked, point farming cheats the system and shits on the efforts of both the middle-ranked teams clawing to be adequately equipped and the high-ranked teams to put in the blood, sweat, and tears to be the best and appropriately rewarded for such.
As for the fear of "rating-scalping," in hindsight, I doubt any high ranked team would willingly sacrifice team chemistry, much less a bounty of points, for a sack of gold to let some lowbie with a cart full of arena points piggyback them for a week and get his S3 weapons. I'd only cycle out for an active teammate who wants in on the action, never for a total stranger.