Are two knights generally worth less than two bishops? A bishop and knight are considered to be about equal in value (three points each). There are even some (closed) positions, where a single knight is worth more than a "bad" bishop, although there are more positions where a bishop is worth a bit more.

But unless they're linked and extremely well placed, (e.g. d4 and e6), two knights are seldom worth more than six points. Some masters put the bishop pair at seven points in an open position because the two bishops together can cover squares of both colors, while moving faster. Does this make two bishops worth more than two knights?

I have read that a fairly open position, two knights may be worth no more than one rook (five points instead of six, or two and a half each). I've never heard this about two bishops. Does this strengthen the case that two knights are worth less than two bishops?
According to GM Larry Kaufman, who has worked on the chess engines Rybka and Komodo, and has done extensive database analysis of material imbalances,

The bishop pair has an average value of half a pawn (more when the opponent has no minor pieces to exchange for one of the bishops), enough to regard it as part of the material evaluation of the position, and enough to overwhelm most positional considerations. Moreover, this substantial bishop pair value holds up in all situations tested, regardless of what else is on the board.

although he goes on to note some exceptions. The whole article is worth reading.