I have a dolly mainly because I am forced to based on the vehicles we own.
Pros: You can tow multiple vehicles with the same dolly (provided they can be) without modification - I've used ours to take a car to the shop, help a friend who broke down etc. They are versatile and usually cheaper than base plates bars and aux brake systems. That's about it for the plus side.
Cons - they take up more space for storage, They are heavy and add to your tow weight,they are a PITA to load sometimes, straps are a little scary, it's another set of wheels and axle to maintain, another set of tires to maintain and risk blowout. Did I mention they are a pain to load sometimes?
Given a preference I would tow 4 down, actually looking at getting another vehicle simply for that reason. I tore the front lower fairing off DW's civic last year when it slipped off the ramp and caught the plastic. $1000 later it was fixed. It takes me about 15 minutes to load the car on a good day and no fun in the rain I assure you. Not to mention you get soaked, the tires slip on the ramps etc and it's just such a pain. Then you worry about the straps etc.
But like I said, it's an option if you have no other and it's more versatile. But I hate using ours. I won't have it much longer.