Skins aren't exactly easy to make. And if they're not made well, they look like crap. The people that make them well would probably want SOME sort of compensation. Sure, the community is great, but it's still a lot of time to do 8 7 teams for the invite level. Admittedly, it'd only be sponsored teams in this discussion, but at that point, what difference would it make? You have Vector and AG in invite that would have skins, which leaves six teams vanilla. Leaving the majority of casts without skins and therefore, advertising.
I'd love skins on teams, representing them in unique and vibrant ways. But at this point you might as well have everybody wear the same hat.

Cute little shirts, uniforms, custom weapon skins branding iT!, logos on fully capped points, and billboards/sponsor sprays on the map would be something I'm sure all sponsors would be interested in seeing.

Just because they're hard to make doesn't mean the sponsor will shrug it off because it's too hard. They sponsor the team.... I'm pretty confident if this was made available in casts and streams, sponsors would spend the money to have a bunch of different skins made.. re-usable after every season...

would pay money to see everybody on xen with all over print arc jerseys

Valve is unlikely to do it for JUST invite players. Their whole marketing strategy works upon an equal opportunity basis.
Personally, I don't support adding more and more cosmetics to the game.

http://i.imgur.com/s0YHnu6.png

Thus, the only options would be custom skins for every match/ team, increasing production time and making caster's jobs that much more difficult. It's a tradeoff.

How about this: Miscs that can be all-class and have logos applied to them a la the sign weapon? Then make it so that they work in sv_pure 2 or on STV.

They're really not as hard to make as you think.

Skins are not difficult to make, aren't they just PNG files? Lay it out and paint a new one.

Also, we aren't asking Valve to be involved, we're not looking for valve items (although that would be kick ass). Why not just have the casters swap out folders in their TF folders before a cast. Takes 20 seconds to change outfits for a team, another 20 to change the skins on the viewmodels.

http://www.extelevision.com/how-to-make-custom-skins/

So more stuff like this, but bigger and more noticeable: http://wiki.teamfortress.com/wiki/Clan_Pride

Bingo, something large enough to put a decent logo on but that doesn't overpower the team colors.

Wouldn't it just be waaaaay easier to throw up some decals around the map on the billboards and around the points and stuff? Then you wouldn't have to coordinate skins, it'd be easier to make, and you'd only have them on the side that whatever team is being sponsored, is defending (i.e. if AG is on red, then there will be AG advertising around their side of mid).

That'd require something to switch the logos around at the half. Over at eXtv we've got a set of logos that replaces ingame signs with eXtv and EOR logos (and Extine's sideburns) all over the place. But in that case you could only splatter the sponsors in a random pattern.