While Tom's spot on in his analysis, there are a number of factors that argue in favor of using some form a single-blind test when auditioning new speakers.
Let's say I go into a store that maybe carries 2 or 3 lines of speakers that have speakers in my price range. Just for kicks, let's just say these two brands are B&W and Klipsch (OK, these are two brands at a store that's local to me).
Now, first, I'm going to hope that the store has a way to compensate for the efficiency of the Klipsch (because, if they don't, it'll quickly be obvious which speaker is Klipsch and which is the B&W: the Klipsch'll be a lot louder).
Let's presume they have this, then what I'd like to be able to do is switch back and forth between speakers without know which one is playing. I don't want to know which brand is playing because all my expectations about that brand will color my judgement. So, a single-blind test, if set up correctly, will prevent me from knowing which speaker is playing, and, therefore, I like to think I'll actually decide based on how the speaker sounds rather than on my expectations about how I think the speaker ought to sound.
Now, when you switch to other things, say, like $15,000 speaker wires, you better take the time to set up a DBT for me. That's a lot of cash to drop on something that, IMHO, will have, at the very best, only a minimal impact on the sound quality.
Um, for the record, I don't have $15,000 speaker wires. More like $15 speaker wires.