In my personal opinion, based on experience, is that a pug formed through the group finder for HM Ops is more likely to fail than it is to succeed.
The story mode ops are pretty simple. It doesn't take a lot of effort to get the group coordinated enough to pull off these encounters. A HM ops requires more coordination, a tighter level of dps and a better understanding of mechanics.
If you're not a hard-core raider, and you don't have the ops on farm already, you are could to have to work at the encounters several times before you can get through it. A pug T/Z will take several attempts just to get the tank swap mechanics smooth enough to pull it off. After several wipes people are going to get frustrated, quit and complain on the forums.
This will result in 437th threads complaining about how impossible it is to do HM Ops through the group finder, with 1,043 responses saying a) you suck or b) learn the mechanics or c) get a guild or d) yeah nerf it it's too hard.
When I first started HM EC, I had a guild and we wiped dozens of times on Toth/Zorn, racking up impressive repair bills. When we got our heads out of our *****es and actually figured out what we were doing, we were then able to progress to Firebrand/Stormcaller (surprisingly easy by comparison) and then Vorgoth. We're not hard-core server-first raiders -- but we eventually mastered that content.
But let's be realistic, you're not going to get hard-core raiders in the group finder. You're going to get the guys that need 37 attempts at T/Z before they figure it out. No pug can survive that.
The reason they wouldn't be able to figure it out if BW just lumped the HM's in is because there would be no progression check. Had a funny experience last week when I resubbed, we had a pug group going for EV HM with most players with at least full columi if not BH/rakata. You'd think it would be smooth sailing but when we got to gharg one of the players who kept on standing under his pounce said he never done this boss before and soon after a wipe three players rage quit.
Instances like that could be avoided if pugs were actually forced to complete the first set of story modes a number of times first before they could progress onto the harder content. Going a privately made pug HM op group is just as bad as the option being in group finder, so why not add it to group finder anyway
and force people to complete the easier content
a number of times before they progress onto the harder stuff?
At least then you'll know that people have completed all the content up until then rather than joining a group blind.