Bill Spight wrote:
Since having fewer groups would be advantageous, stone scoring would, I think, encourage building large frameworks. Also, playing a framework game may well be easier for Black, so that komi might increase.
Not really, for old Chinese Go rule set.
There are 4 stones placed at the 4 corner stars(星，xing，hoshi） before game starts, 2 black stones at 2 opposite corner stars, 2 white stones at the remaining 2 opposite corner stars. So there are four separate groups even before the game starts. It would be hard for the first player to build a big framework. The advantage of first player should be smaller than that in a modern game. Perhaps that's one reason for this rule, because ancient games don't have komi.
Other reasons could be,
Prevent mimic games
Encourage fighting. There are 4 different groups right from the beginning. This tends to increase chance of conflict. Group tax also encourages cutting of opponent's groups.
Modern games may seem free. But people seldom play opposite corner openings these days. It's a natural choice, because you get higher efficiency by placing more stones on one side. It may not be so free as the first sight suggests.