EJ's also been getting much more vanilla looks, Kolb isn't - and it's a lot easier to look good when the D isn't blitzing or disguising things as much, is it not?
I think it's nothing more than a HC and OC being smart - they're bringing EJ along slowly, letting him experience success, gain experience and confidence, and gradually ramping up the degree of difficulty to ensure his trajectory toward a starting role is a positive experience.
How many times have we seen young QBs get frustrated and start to push too much and/or try too hard? Inevitably, they get even more frustrated because doing that doesn't lead to success, and they try even harder and push even more - it becomes a vicious self-defeating cycle that is hard to break.
I'm sure Marrone thinks EJ will eventually be the guy, but I sincerely believe the Week 1 starter is not predetermined. I think it will be the QB that he feels gives his offense the best opportunity to succeed. I don't feel, from Marrone's perspective, that there's some master plan beyond what I outlined above.