it depends on the position. what is my advantage besides an open file. do i have an attack after the trade. what can black do to stop me after the trade off?????
Exchange a Rook & a Pawn for 2 Bishops.
I prefer the 2 Bishops. The pawn can only be used in the endgame and the Bishops will often easily disrupt the rook from using the files.

If you look at it from the points perspective its 6 for 6, so its even in that regard, you just have to look at the position and judge for yourself what is going to make your position better

This theory are purely tactical. But I believes, if you have a strong play in the middle game, it's going to be a great advantage for the end game.

Sometimes one side sacs a rook for a minor piece in the early middlegame, relying on the activity of his minor pieces and the inactivity of the opponent's rook. An example of this is the thematic sacrifice on c3 in the Sicilian.
R+P v. B+B: It really does depend on the position. Ask a more specific question and get a more specific answer.

Two Bishops are definitely are better than one Rook. In the middle game, if it means to sacrifice your pawn for a strong centre play, I think I'll trade my Rook and a pawn for 2 Bishops. The impact are very obvious, now you have 2 Bishops in your hands to initiate some dangerous moves.

two bishops are clearly better than a rook and a pawn, maybe even a rook and two pawns, in all parts of the game and most positions... so i would definitly get rid of my rook for my opponents bishops.
Let's say, when you are going into a middle game, what would you rather keep (apart from other pieces). Would you rather exchange a Rook and a Pawn for 2 Bishops ?