I would imagine that part of the CTC's argument could be that the more posties there are on bikes, the more cyclists there are on the roads, and as we know, the more cyclists there are, the safer it becomes for cyclists.
I love seeing posties on bikes - there's a bunch of 4 who cycle up past my workplace in formation, and very jolly they look too.
The union chap sounded to me like he'd made his mind up and just didn't want to listen to alternatives - as Andy said, he seemed convinced that bikes were backward.
As to how much one can carry - well, with a Maximus trike I'm regularly hauling 100kg or more of recycling. Our cycle couriers in York use Maximi, Yuba Mundos and 8Frieghts, and carry plenty of stuff.
Also, you can push a laden bike or trike down a pedestrian street, at times when you can't drive a van down there.