At some time or another in their career, very likely. It varies based on where they work, of course. Usually, teachers in big cities will fare better than teachers in small towns and suburbs. But it's a common thing, and it seems to be getting more common.
There are two sides to this. One is the effort by creationists to teach some kind of religiously based idea as part of the science curriculum. That's usually pretty blatant. But there's another side, which can be a lot harder to see. Teachers get the message, sometimes overtly, sometimes more subtly, that evolution has become a controversial subject in their community and they'll just quietly stop teaching it, and evolution will sink out of the curriculum.
At its heart, the "equal-time" argument is substantially flawed. People who advocate it are basically saying we should teach that evolutionary theory -- the idea that the universe changed through time, that the present is different from past -- is equal in weight to the idea that the whole universe came into being at one time and hasn't changed since then. You can't do that in a science class. You can only deal with scientific evidence. There is copious evidence to support that evolution has occurred, and no evidence that everything was created at once and hasn't changed. Why would we pretend that an idea that was created outside of science is science? That's not fair.
It's perfectly reasonable to expose children to religious views of origin, but it's not OK to advocate those views as empirical truth. And the place for those ideas is not in the science curriculum.
Yes. To begin with, these so-called alternatives to evolution are disadvantageous because they are simply factually wrong. Creation science literature is riddled with inaccuracies, misstatements. Students who learn it learn a lot of flat-out erroneous stuff. They also aren't learning the scientific method. The people pushing creation science aren't interested in modifying or revisiting their theories based on any new evidence, which is the basic premise of science. So when you teach creation science, you're giving legitimacy to very bad scholarship.