You can stretch in tai chi forms, but it seems to me that it could be missing the forest for the trees while simultaneously giving a mediocre stretch compared to a proper stretching session. I don't know if its a universal, but as I learned it, tai chi is about walking a fine edge- not too much, not too little. One shouldn't sink too much that it disrupts the structure, or too little that its not involving the legs enough. It should be just the way its supposed to be (this contrasted with some of my kung fu training that overly exaggerated some things). While the middle of the road approach is good for some things, stretching doesn't seem like such a great one.
FWIW I did Yang style, but I've trained with Chen style folks as well, and I know that they tend to stretch down more and get very low.
At any rate, the kind of stretching fostered in tai chi forms is different than the type developed in yoga.