Well, one use of them is for outland/northrend gear, as you can't use Cataclysm gems for those.

Like he said, they are the best gems for pre-Cata gear. Of course, not a lot of gear people use outside of a small spurt at 60 have a lot of gem slots. So not a big market, but a tiny niche for 1-2 sellers per server.

Scarlet rubies are an amazing seller - I can shift the cut ones (+16 agi/str/int) for 50g+, and they almost always sell.

The reason for this is lvl 60+ pants - here's an example:

http://www.wowhead.com/item=24046

There's something similar for all classes from the early BC Hellfire dungeons with 3 gem slots. 3 wrath-level main stat gems in there (you can't use cata gems in gear less than iLvl 285) makes them extremely OP, up to and well into wrath content.

Add to that the fact there's no heirloom pants yet, and these are the choice of the well-funded levelling alt!

Technically the epics (+20 stats) would be better, but they are in very short supply, and no-one blows their alchemy cooldown making those any more.

I would think they would always hold more value to crafters leveling up their skills. I got a bunch of dragon's eye a couple weeks back and sold them for about 95g a pc. I still do the Dal JC daily on an alt who is still leveling up in northrend. For some reason the dalaran fishing and JC dailies give top gold and XP while the cooking one gives substantially less.

I actually can sell some Wrath gem cuts at a pretty good rate-- any red mainstat and stamina gems are by far the most popular. However, BC socket bonuses are tiny and usually really odd, so cuts people use to match sockets, like haste/str, usually don't go.

Let's not forget BC and Wrath Engineering goggles. They usually have a meta and another gem slot. The Leather Cenarion Expedition chest you can get at 62 has 3 red gem slots. There's also a JC AGI neck with 2 red sockets. All very useful.