This question/request comes up frequently, especially when the question has several applicable responses.
Why can't we choose more than one pollbooth answer?
Well, Dave would have to tell you the technical reasons why we can't.
From a practical standpoint, the pollbooth isn't meant to accurately/scientifically measure anything - it's just a way to get us conversing about a topic, and it's kind of fun to know if you're in the majority, or "odd man out" on various topics. Have you noticed how sometimes a person you might have thought you have nothing in common with leaves a comment that is exactly what you might have written? Poll questions can draw us closer together, and let us find out more about each other - things that we might not think to tell one another unless somebody asks.
Since non-subscribed members can also participate, it's often their first interaction with DG subscribers, and hopefully they see we're a pretty fun, lively bunch of gardeners here! ;o)
Here's the technical reason:
Short answer: It would take much more time to program.
Longer answer: When I first made the voting booth, I didn't think anyone would ever want a multiple-selection capability, so I programmed it to only accept one answer. To change that now would mean almost re-writing the voting booth from scratch, which I really just don't want to do, since it's working so well. So,it's a matter of priorities. :)