Don't cut quite as well? I've never heard that. That being said, my dad's new Snapper commercial with a angled front on the fabricated deck does cut better than any deck I've tried on any mower. I think the angled aspect let's the grass stand back up before it gets to the first pass of the blade.
They were referring to nice clean even cutting and efficient mulching. You can do more in the design of a stamped deck as far as airflow and such. At least from a manufacturing cost standpoint, nobody would want to pay for a fabbed deck welded from 100 pieces. It sounds like they are gaining ground tho.