HOMM 3 and M&M 6-pack are consistently on top-10 bestsellers every week.

Bad example. What about HOMM and HOMM2? What are their sales figures?

Never see them on top-10 bestseller, as far as I know. And no DD store ever released their sales figures.

If I had to guess, I'd say that most people bought HoMaM1 while it was still $5.99...
IMO - it would be quite fair if they priced HoMaM2 @ $7.99. Sure - it's quite old, but it's a good title and for that price we actually get the expansion pack, so...
Still - the first one should fall back down to $5.99. It's fun but it just doesn't seem right to pay the same price for Heroes 1 as for Heroes 3 with 2 expansion packs...

The way I see it, I like GOG because I like getting good old games for very reasonable prices without any hassle. Of course, this is just a subset of my desire to buy things I like at reasonable prices without any hassle. If GOG eventually expands to encompass newer games while still providing prices and a lack of hassle that makes the whole thing a good bargain then I really can't see how that could be interpreted as anything other than a Good Thing (we'd still be getting good old games as a good bargain, plus we'd be getting newer games as a good bargain as well).
I'm personally a pretty cynical and paranoid person, and although it may just be my ego talking I think I'm also pretty good at seeing through corporate doublespeak. I just haven't any of that kind deception and doublespeak coming from interviews of GOG and CD Projekt employees. Make no mistake, they're running a company and are looking to make money, but they seem to recognize that the way to do this is to offer deals that are beneficial to both themselves and those they are doing business with (e.g. us). The way that GOG and CD Projekt acts in the way they do business also supports what they say. Thus I'd say that the idea of introducing additional price points should be taken in the context of these considerations.

I trust them, I'm just a cheap bastard. :)
But I do think it would by nature open things up for some degree of abuse by publishers, even if only a bit.

I don't think GoG will ever be preasured to increase their price just because others are. That just means GoG's game is a better value and that is the whole point. In fact, others will be preasured to lower there price to compete with GoG. One thing can be certain. If GoG adds games for say $12.99 or $14.99, they will make sure you have value for your money and this is precisely their areas of expertise. So no worries here.

I'd like to add GOG would lose a significant amount of uniqueness if a higher price point was introduced. One of the main selling points is, "For less than the cost of a lunch.." Higher price = considerably more like every other service.
Also, I kinda think GOG should give higher sums for catalogue rights instead of passing costs on to us, if there must be a choice between the two. Given how much is saved due to there being no manufacturing costs...kinda the ethos of digital distribution. Or should be.
And finally, they've said a lower price point would be introduced...I'd rather pay $6 for say, Personal Nightmare (which I did), than $3 or $4 and have to pay $15 for other titles. And there's always sales. But that's me...

I'd rather pay $3.99 for Personal Nightmare and $12.99 for Planescape: Torment.
Remember - the only games that will get priced higher than .99 are the ones we wouldn't get at all.

Planescape is a very old game that is gaining the parties involved zero revenue. 10 should be enough.

it retails in Europe for the equivalent of $20 USD.

It retails in Europe? Holy crap. Well, in that case you've got a point. But I still think that's way too much for such an old game.
edit: I can't find that game online anywhere. Also, wasn't Arcanum also going for ridiculous prices?

here's a link to the US amazon http://www.amazon.com/Planescape-Torment-Pc/dp/B00002EPZ2/ref=sr_1_1?ie=UTF8&s=videogames&qid=1264953592&sr=8-1
just to note, it comes with a PEGI 16+ sticker, so it's definitely for sale in Europe. I have a copy and it went on sale during the holiday for $0.01 USD with free shipping, many people got it, many people did not.

Planescape: Torment, on Amazon UK, for £13.

Ah, that's pretty cool actually.
But those 3GBP could easily be ascribed to the fact that it has a manual, comes in a dvd cover etc. You know, all the material stuff. And apparently this version is already XP and vista compatible so there's little for GOG to enhance.