I have to disagree a tiny bit when it comes to Forbidden Planet. Anne Francis' character in Forbidden Planet got poor dialog, it's true, but so did everyone. Her character was, however, in control of many of the scenes she was in. It was the male characters in those scenes who got the most stoopid lines. Her character was clearly smarter than any of the men except her father. She was also innocent of all evil.
The overall point, that women in science fiction get second-rate parts, is valid, I think. A few TV series now in production seem to be trying to change that. The SciFi Channel's The Invisible Man has a credible female scientist, for example.
Movies, as a whole, have not treated women particularly well (as compared, say, to books or plays). Why this is, I don't know.