robsta

Hi
As you tend to ride a commuter bike the most I would imagine a folder would be an acceptable replacement. However, it may depend on your style of riding. If you like to be out of the saddle and pounding on the pedals a folders might feel a little strange. The smaller wheels may give an odd ride but you will soon adapt.
For me, the key to any comfortable bike is the right fit. If you are in no rush try to ride (or at least sit on) as many folders as possible and see which feels right.
I only use a folder these days. For me, the Xootr Swift has been a great buy. I can use it for a cardio workout, tour or commute. The Brompton has a great fold and Bike Fridays are very well regarded. Dahon has a great range of quality and price.
The tricky part is not CAN a folder be your only bike but WHICH folder will you choose.
Of course, you are asking in the folder forum so expect slightly biased answers. I wonder what reaction you would receive from the road forum?
Good luck
Rob