Well, I don't see why T-Mobile is the only company in Germany that is able to make everything work properly. Apple could easily team up with other providers and make sure that they provide the same quality of service as T-Mobile (in fact, it's not really a technical problem, the features required by the iPhone are so dated that every provider in Europe supports them).
I personally don't care who sells the iPhone because I don't want one, however, the argument that sticking to one provider improves the quality seems flawed to me. Simple (hypothetical) example: I might happen to live or work at a place where there's no T-Mobile reception, but which is covered by Vodafone or O2 or whatever. I might still want an iPhone, even though you think I'm not entitled to have one, because I'm not the CEO of Apple. How am I better off now?