What exactly would Bonner do as a Mav to lose game 6?
uh...play in it doofus

Just his presence alone would of motivated Lebron, Wade and Bosh to drive towards the rim. Remember that Haywood was injured(and like many predicted Tyson to be in foul trouble), so Dallas wouldn't have any other choice other than put the season and championship on the line into the hands of the ginger and dirk guarding the rim.
The Heat actually did have a field day when Mahinmi tried to guard the rim.
I don't see much difference there.
Mahinimi at least gave an effort(tried to contest shots) and knocked some fools down when they tried to drive towards him. He scored a bucket, grabbed a few boards, and his points contributed to a mavs victory in game 6. Can't say that Bonner would of had the same impact.