It's not just die-cast, MP01 is a significantly more detailed and complex toy. Being originally designed to be just a cab/robot, they really poured a lot of love into that toy. MP10 is a notably simpler figure by comparison, but it's still an excellent toy, and as you rightfully pointed out, it does have a great trailer and Roller which MP01 never had and MP04 didn't have Roller. I find both MP01 and MP10 to be great toys in their own rights, each bringing different things to the table. MP01 is a superior robot, but MP10 is better as an 'overall package.' :)
I only ever had chipping w/ BT1 Smokescreen, nothing much after that. I once accidentally dropped BT Dead End on the footpath while walking down a busy city street and there were no chips. :o
IMO die-cast enhances the look of vehicle modes. The great thing I love about BTs is how they look like miniature replica cars, something that I felt was lost with Alternators; partially due to the cheaper looking plastics that Hasbro chose (unlike say the plastic quality used on Alternity). So plastic itself can still look close to authentic if done right - but it still doesn't look or feel as good as die-cast (and chrome!) IMO. :)