Don't get me wrong, there are plenty of situations in which it is reasonable to expect an opinion. However forming an opinion in the absense of data is unreasonable. Lets look at some examples.
- Is linux ready for the desktop: Hell yes. I'm using it myself, at home and at work.
- Which is better, mysql or postgresql, for this particular task: I have used mysql, but not postgresql. I can do some research, consider features, read whatever has been written on the subject, and then come up with something that resembles an opinion. Actually what I'd have would most likely be somebody else's opinion, or a combination of several somebody elses' opinions. I am fairly well suited to this task, due to a background that includes related experiences.
- Which pair of high-heel shoes do I like: How the hell should I know? Where is my frame of reference? I've never worn high-heels, and have no idea what makes one pair better than another. I'm not even very well suited to researching this, since I'm pretty sure they won't make them in my size. Hell, most of the time if I ask the shop assistant which one is better I'll get some wishy-washy answer about taste and style - and this person sells the damn things. What hope do I have?
- Do I think this kooky new-age course is a good idea: I could probably research this, as in 2, and give you somebody else's opinion on the subject. But you can do that yourself, so why the hell should I do it for you? I have zero experience in the area, and there is no reason to expect my research to be better than yours.