Against the Titan
Here's an illustration of problem:
The relevant stats for a titan: Grapple AC 34, Grapple attack +44, Touch AC 8, STR check to avoid trip +24.
I think we can see from the fact that the Titan's grapple modifier is 10 higher than its grapple AC that Morrus' system skews heavily in favor of the attacker, but let's contiue with this example
Let's suppose a 10th level figher with 22STR. He started with 16 - just a little nudge from the elite array, and has +4 from a belt and +2 for the ability increases at 4th and 8th level. He also has the improved trip feat, for a total modifier to his tripping check of +20.
Under the core rules, his chance to hit the giant is a compound probability problem. I'll keep the math in parentheses.
First, the fighter must hit the Touch AC, which means not rolling a 1.
(.95 times the sum of...)
Next, the opposed roll - the Fighter succeeds if:
The titan rolls 1 and the fighter rolls 16-20 (.05*.25 or .0125 plus)
The titan rolls 2 and the fighter rolls 17-20 (.05*.20 or .01 plus)
The titan rolls 3 and the fighter rolls 18-20 (.05*.15 or .0075 plus)
The titan rolls 4 and the fighter rolls 19-20 (.05*.10 or .005 plus)
The titan rolls 5 and the fighter rolls 20 (.05*.05 or .0025)
... which comes to a little more than a 3 and a half per cent chance of success.
Under the Grappling for Beginners rules, the Fighter succeeds 35% of the time - on a roll of 14 or higher.
However, my proposed revision (adding BAB to the Grapple AC) doesn't preserve the minute chance that the fighter has in the core rules - he can't hope to hit the Titan's grapple AC of 54.
If we go back to an opposed roll system but eliminate the touch attack, the fighter still has no chance against the Titan, but it does increase the range of situations where the underdog has a shot. If we pick an example Giant or another big creature with a CR closer to 10, the GfB rules still give the Fighter a much better chance of knocking things down than they get in the core.
Many thanks to my friend Tom for noticing how the GfB rules favor the attacker significantly.