The User is Not a Designer
As freelance graphic and web designer, I quickly learned that my clients were not designers. After all, that's what they were paying me for. As difficult as I sometimes found it to decide amongst subtly different logo variations I'd created, I knew that throwing them all at my client and asking "which one do you prefer" wouldn't turn out well for anyone.
The same applies even more strongly user interface design. The role of the designer is NOT to mock up 3 different layouts and then ask someone else, be it her boss, the CMO, or her end users, which one they "prefer." It's just...irrelevant. Now the good news is, unlike with a logo, there are ways to put a UI design to the test.
I have been using usabilityhub.com a lot recently, experimenting with click tests and five-second tests. You can earn responses to your tests by taking others' tests (awesome), and I always find it interesting to see what other people are testing. I have noticed that a lot of people create click tests containing multiple variations of a design in a single image. The instructions are usually something along the lines of "click the one you prefer".
Instead, see which actually performs better
A better approach would be to focus on one function the design has to serve or piece of information it has to convey, and set up a test with each design as a separate variation. (Luckily, UsabilityHub has this functionality built in quite brilliantly).
For example, in a list of products, users have to be able to add the quantity they want to the cart. So you could set up a click test with multiple variations, and ask testers where they would click to add 2 widgets to the cart. After the results are in, you can compare them side-by-side to see where people clicked, and which was most successful.
There are certainly contexts where "preference" is valid. As Jakob Nielsen points out, if users have had a chance to really try out two different interfaces, and thus truly know which one works better for them, their preference is important. More specific questions than "Which do you like better?" could also provide good feedback, such as "Which logo says 'eco-friendly'?"
But, to paraphrase George Bush: "I'm the designer, and I decide what's best (based on careful observation and study of user behavior, as well as years of experience and schooling. Also, I'm willing to change my mind based on evidence. That is all.)"
First Rule of Usability? Don't Listen to Users
Three Questions You Shouldn't Ask During User Research