This is NOT a question of "ab initio vs GA"!!! This is a question of the TOTAL airplane experience and "air sense" a pilot has after 50 hours in an airplane vs after 250 hours in an airplane!
One does NOT "improve" training by reducing the amount, intensity, or thoroughness of the training. One does NOT improve training or experience or air sense by replacing X hours of actual instrument time with X (or X-y) hours doing (or observing) canned procedures in a simulator.
The MPL will put pilots in the right seat of a 737 or A320 with MUCH less time and experience -- especially instrument time and experience -- than a CPL/IR pilot!
It's also interesting to me that the JAA and CAA, who currently have probably the most rigid testing program in the world for the ATP license, would agree to allow "untested" pilots in the cockpits of commercial airliners. It gives evidence that the entire testing program is a sham!