Reasons I think he had:
Modern day compact cars don't represent the character of BB in any way other than they're small. A crappy aging vehicle that can't keep up with modern day engineering does. A decrepit Camaro looks more out of place next to modern cars, trucks, tanks, jets, and so on than a brand new Toyota Yaris or Chevy Aveo. If you see these modern cars, with a crappy beat up Camaro in the mix, you look at the Camaro and think "what's that doing there?" Just like you'd think of BB when it comes time to oppose the Decepticons.
Additionally, while perhaps it would have made sense to choose another aging car model from around the same time frame to insert instead of the Beetle, I think every one of us would have sighed if Sam's new car/bot was a K car. What the beetle had that those other small cars didn't was personality in spades. Sam isn't going to fall in love with a ****ty K car. He is going to fall in love with a Camaro, no matter how crappy it is, because it can be so much more. Again, just like BB. The beat up wreck of a car that'll rise above his shortcomings to be great.
But hey maybe none of that stuff counts...