I think the majority of gamedev readers would prefer most articles to focus on practical fact rather than opinion, with a number of sites such as GamaSutra providing excellent avenues for distributing opinion. That said, I don't think that this should preclude articles that present an opinion. I support clearly identifying these articles, and an author should declare the context surrounding their opinion.
I think the most important determining factor for the suitability of an opinion article is its substance. Articles presenting an opinion should be presented, effectively, in essay format. The author should be introducing the topic and context. They should then present a well supported argument for the position, complete with examples or citations, informally or otherwise. It comes down to presenting an opinion that can be justified and evidenced.
If an opinion is explained, then it can be discussed intelligently and it practical lessons can be taken. If an opinion is expressed without context or evidence, it effectively becomes a soapbox or rant, which have their place in blogs.
An example I may offer is a discussion on piracy. These discussion happen often and they're rarely intelligent or worth-while. If someone presents a lengthy opinion arguing that piracy is destroying the industry it benefits no-one and will quickly devolve into back and forth. If someone puts forward their opinion that piracy is a more significant issue than we consider it to be, complete with citations on sales and the results of various studies, possibly summarising with a proposed approach, then there are details that can be discussed and lessons may be learnt. It'll inevitably turn into bickering anyway, but it at least has merit.