...Not sure how well I've explained my thoughts in this rambling on, but hopefully it adds to the discussion a bit.
Those are valid questions and a valid point of view. But I suppose (based on common sense only, since we don't have statistical data to crunch) that the 30% figure first appeared the other way around. Id est, there was a corresponding increase in sales after the publisher implemented some form of DRM (in my case, it was 1C and Starforce). Of course the increase can be attributed to any kind of reason, like quality of the product, the market situation, the date of release, advertising etc, etc. But if you experience a stable increase of sales all over the product range after some action, you tend to think that this action is the cause and, importantly, chances are, you are right.
I'm personally not inclined to give any more credence to such an opinion voiced by an industry members as opposed to any random schmo on the internet. Actually, I'm inclined to give even less credence when an industry member voices such an opinion. This is because there are so many within the industry invested in the idea that DRM helps sales. When people have been invested in a course of action for some amount of time they want to believe they've been doing the right thing, so their perceptions of the world are filtered and shaped around the idea that their course of action was necessary and proper. Additionally, there's a strong perception within the industry that DRM helps sales, and this perception will color the interpretation of any information surrounding the matter. All together it's basically a non-scientific version of an experimenter's bias.
I have no doubt many folks in the industry do feel it's necessary; the continued use of DRM is pretty much irrefutable proof of that. What I'm questioning is whether that feeling actually corresponds to reality. If it turns out it does I'll happily shut up about DRM; I still won't like it, but I'll accept it as necessary. However, when someone (anyone) makes a claim like "DRM results in 30% more sales" I want to know how they actually arrived at the conclusion; without the access to the information and thought process that led to that conclusion then it's just another baseless opinion as far as I'm concerned.
The argument about bias is strong. I think we all know how resilient an entrenched wrong theory can be. However, you have to admit, you are biased, too. You are inconvinienced by certain measures that publishers use to prevent immediate and all-encompassing piracy and you are against these inconviniences. Furthermore, you see no direct benefit from DRM, so for you it is all there just to hinder your fun with a product purchased and used in good faith. Judging by your second paragraph, you are reasonable and can even live with it if you obtain conclusive evidence that there is a deeper meaning for DRM.
Which takes us to the other problem. The only source of data we can have is from the industry and you distrust that source. So there's nothing which can make you "happily shut up about DRM".
My common sense tells me that DRM really is an unavoidable evil, unless and until they find a way to personalize digital data copies (sort of like Steam does, but obligatory), which will be a DRM in itself. GOG is a great exclusion to this rule, however, as it was mentioned already in this thread, it will never work with new titles. With all due respect, GOG is about giving a second commercial life to software, which has already lived a thrilling first one.
Your common sense tells you otherwise. Great.
Edit: Oh, and you get a + for not being like tb87670