I have a slab-sided Colt configured as an A2 Tactical Carbine (with the larger M4 grip).
I also have a "fenced" A3 Tactical Carbine.
They both shoot great and have incredible quality (fit and finish) compared to other ARs that I've seen.
It is my understanding that the fencing was added to minimize the possibility of snagging the mag release on brush. I've heard from other slab-side Colt owners that they have never experienced this, and unless you were slithering around in a Southeast Asian jungle I wouldn't be too concerned.
As far as strength is concerned, I periodically carry an M-16 that is an A1 configuration and it is slab-sided with many thousands of rounds through it. So, I'm personally not aware of a durability issue.
Hopefully, folks on the board with more knowledge than me on the subject can correct any errors or omissions I've made and can elaborate...