Uh... so what does define a professional game tool, Jack?
Either way, yeah, companies are(or will be) using GM-Studio, they even hire:http://yoyogames.com/news/96
Well, hm. First of all, "professional" is often quite ambiguously defined. I understand your defensive response because I feel people often strongly equate "professional" with "good", which need not necessarily be the case. I was merely using the OP's point about it being "professional" to make a criticism of their continual raise in price.
Now, my personal beliefs. I think it's rather silly to call a tool "professional". I usually attribute the term "professional" as a descriptive of someone who is in the business of making money at what they do, which certainly many people do using Game Maker. But the notion of a tool itself bringing in money is ludicrous. Is the tool suited for professional use? That's ambiguous too, because needs of "professionals" vary widely in this industry in particular. It's more acceptable to call a camera "professional", because most professionals will converge towards wanting the same quality(that being high) camera. Less so when it comes to developing video games.
If companies choose to use Game Maker, cool. I hope it works effectively for what they're trying to do. There are lots of companies who make small games, so I think such a tool could "technically" be quite viable. I do have criticisms of the software though, and I do believe them to have objective grounds. Ultimately the tool isn't optimal for large game projects, and it's absolutely terrible for optimal-performance oriented programming. While Game Maker is very flexible and very capable, I would not consider it optimal by any means.
And I stand by the notion that it's very silly for Yoyo to try to market this to "professional" work environments. Any professional company will most likely hire experienced programmers who won't wish to tolerate the nuisances of development with that tool. Game Maker is wonderfully optimal for getting things done fast, but not so much for getting things done 'well'. So by approaching this medium market of small business who make small games they're limiting their scope incredibly, particularly considering they are starting to create features for this market which they explicitly deny to the more casual audience; effectively alienating the user-base which made them who they are. Other tools are far more effective and do not come with a price tag. If you're looking for a tool that's best for making a quality product, Game Maker is probably not the tool for it. Yes, Game Maker "can", it's highly extensive. But it's not optimal. Start doing anything highly technical or divergent from Game Maker's way of things and all of a sudden you're in for (highly unnecessary) pain.
That is my stance.