How beneficial is it to swap bikes? Today I finally got a couple of Urban Cruisers I ordered a while ago.

For the last 7 months I have been commuting on a very heavy cheap 29er mountain bike. These new ones feel incredibly light by comparison, comfortable, rolled beautifully, brakes that actually work, and on my test drive I was flying around, but all in all it was a bit unsatisfying, it was just too easy and it felt 'twitchy'. Skinnier road tyres, lighter frame.

Should I switch bikes every so often to build up my leg strength faster, or will these new bikes do the same thing anyway? I only ride on the road.

More importantly, could I actually lose some leg strength going with the lighter bikes (I didn't feel I got anywhere near the workout I usually do)? Or am I just nitpicking because I'm not used to them?
Bike weight and performance is only one factor.  Your average speed and whether or not you have inclines would also factor in, of course. You can have a light bike with skinny wheels and burn as many calories by averaging 18mph, or a heavy bike and burn the same calories at a lower speed, etc...  Coincidentally, this month's Bicycling Magazine has a brief illustration on how much power is saved by tweaking tires, helmet, clothing, etc...