Many people have argued that the Rangers take many of the core fighting soldiers out of the regular units, which decreases their overall will to fight. This makes a lot of sense, considering that something like 90% of the fighting is done by 10% of a unit.
Also, you wouldn't use special forces to assault a tank column; that's just stupid. Special forces aren't Rambos, they have specific talents that can be used to augment and enhance the performance of regular forces, but aren't a replacement for the normal troops.
The closest thing we have to special forces in the WC series are the Black Lance, which were a highly elite group with special equipment. The elite squadrons that you usually fly with in the WC games are more along the lines of just the best group of pilots on board, and aren't really meant to be elite units designated only for specific special missions. Otherwise, you'd spend most of your time sitting in Pilot Country, waiting to get called up while everyone else flew off and did the drudge work.
*edit*
I also forgot to mention those various special missions you get sent on in the various WC expansions. I think this is most obvious in WCSO, when you're working with a small special ops squadron, but there other instances, like whenever Paladin shows up post-WC1 in his Free Trader. Not exactly relevant to the question about a wing commander sorting out his newbies and veterans, though.