I was never told by a dr. that it was bad, but read it in some of my books and online (spinning babies, I believe, as well as some mw sites) as a way to encourage correct positioning of the baby. It had something to do with discouraging improper stretching of certain uterine ligaments....the same reason they say that it's best to do certain stretches or sit on a birthing ball in the third trimester. Reclining positions make it more comfy for the baby to be in improper positions (not that it guarantees they will be). More upright positions encourage the "correct," head-down position of the baby.
Ds3 was breech several weeks before labor, and I sought out as much information as I could about things *I* could do to make sure he changed positions...avoiding recliners, at the time, seemed to be a popular suggestion. Given all the stretches/posture techniques that were also suggested, the recliner thing made perfect sense.
Just wondering when that starts...