I'd actually split "Vehicle Form" up into different sub-categories:
Street Vehicle (cars and trucks, panel vans, tractor trailers)
Work Vehicle (construction equipment)
Fixed Wing Air Vehicle (planes)
Rotary Wing Air Vehicle (helicopters)
Military Vehicle (tanks, APC's and the like - jeeps and Hummers would be under Street Vehicle)
Motorcycles (Erm, well, motorcycles)
Why? Because, for example, I prefer Street Vehicles to Work Vehicles. I like FW Air Vehicles, but a RW Air Vehicle has to work hard to impress me. And so on.
"Machine Form" TF's aren't always machines, either - let's not forget the pretty durn awesome Longview (
http://www.seibertron.com/transformers/ ... view/1100/ ). Anyway, I actually think that while some might be okay, there shouldn't be too many - if the alternate mode isn't capable of doing something the bot is without transforming, why would he ever transform except to conserve space?
And the worst thing about MOST Beast forms is they all have the same remarkably lazy transform - "Stand up, flip head down or back, stick tail in hand as weapon." Yes, a lot of the more 'legendary' figures don't have that scheme, but start surfing through the Beast War era galleries, and you see FAR too much of it. It's one reason I can't stand most Beastformers.
And there should ALWAYS be a robot form, otherwise it's not Transformers, it's MASK.