If ID really were a scientific theory, positive evidence for it, gathered through research, would fill peer-reviewed scientific journals. This doesn't happen. It isn't that editors refuse to publish ID research. There simply isn't any ID research to publish. Its advocates bypass normal scientific due process by appealing directly to the non-scientific public and - with great shrewdness - to the government officials they elect.
The argument the ID advocates put, such as it is, is always of the same character. Never do they offer positive evidence in favour of intelligent design. All we ever get is a list of alleged deficiencies in evolution. We are told of "gaps" in the fossil record. Or organs are stated, by fiat and without supporting evidence, to be "irreducibly complex": too complex to have evolved by natural selection.
In all cases there is a hidden (actually they scarcely even bother to hide it) "default" assumption that if Theory A has some difficulty in explaining Phenomenon X, we must automatically prefer Theory B without even asking whether Theory B (creationism in this case) is any better at explaining it. Note how unbalanced this is, and how it gives the lie to the apparent reasonableness of "let's teach both sides". One side is required to produce evidence, every step of the way. The other side is never required to produce one iota of evidence, but is deemed to have won automatically, the moment the first side encounters a difficulty - the sort of difficulty that all sciences encounter every day, and go to work to solve, with relish.
What it does is to falsely cast evolution in light of an inherently atheistic idea. This is the goal of the intelligent-design movement, indirectly to tell students that either you turn your back on the faith that you've been brought up with in order to embrace the scientific mainstream, or to be true to your faith you have to reject modern science. That's a false choice. It does disservice to religion, and it does disservice to science, and I think it is a terrible way to proceed with scientific education.
Intelligent design is not a scientific argument at all, but a religious one. It might be worth discussing in a class on the history of ideas, in a philosophy class on popular logical fallacies, or in a comparative religion class on origin myths from around the world. But it no more belongs in a biology class than alchemy belongs in a chemistry class, or the stork theory in a sex education class. Similarly, in a class on 20th-century European history, who would demand equal time for the theory that the Holocaust never happened? In those cases, the demand for equal time for "both theories" would be ludicrous.