My take on Barkley vs Robinson:
In worst case scenario from DRob standpoint Robinson's value on defense = Barkley's value on offense. But even limited Robinson's offensive value >> negative Barkley's value on defense. That's why overall Robinson was more valuable player.
And BTW, Barkley missed many games during his prime, so here's how much he improved his team offense (I looked at seasons when he missed minimum of 10 games and was leader of the team, so no Rockets seasons, which were past his prime, so don't matter much anyway):
1987 76ers offense was better +4 with Barkley
1991 +2.4
1994 +6.7
1995 +5.0
So it's very good, but Barkley's negative impact on defense was so big that in these seasons his overall impact (net) was:
1987 +2.8
1991 +1.2
1994 +8.1 (this season seems as aberration, because it's the only one year when he had positive impact on defense)
1995 +0.2
(all these numbers are calculated from b-r team game logs and I used b-r possessions formula)
I'll also quote what mysticbb said in one of previous threads:
In 1991 and 1992 Barkley missed 22 games, the 76ers went -1.5 in those games. With Barkley during the same time they went -0.6. That is a VERY small difference someone made who had a 26.6 PER. The 76ers were 3.6 points better offensively, but also 2.7 points worse defensively with Barkley. That is not surprising at all, Barkley was great offensively, but bad defensively.
So once again. We had one player great offensively, but bad defensively and other player great defensively but average (in worst case) offensively. Overall value of second player is bigger.