Well, if I were 1500~, the Exchange Spanish and the Rossolimo would not be my very first choice of systems.  I doubt if the OP's game is up to playing those.  He should play instead the Scotch or the Italian and either the Smith-Morra or the Open Sicilian with the KB on e2.  But to see why exchanging the bishop sometimes make sense, he might take a look at the lines of the Old Steinitz Defense to the Spanish, where it's useful for gaining a tempo and avoiding more comprehensive exchanges.

I agree with MNb that you should not exclude from your games and/or studies any moves that may 'feel' bad to you because you cannot understand their meaning as of yet. Of course when you play you cannot experiment all the time, but still you need to occasionaly try out new staff. More importantly it is important to try to understand why some standard moves or plans are played (imo even when it comes to such questions like yours which has no clear answer even among GMs).

I'll give you an example arising from the Spanish Opening (exchange variation):
1. e4 e5 2. Nf3 Nc6 3. Bb5 a6 4. BxNc6
White's main trump is the superior pawn structure: If he/she manages to play d4 before Black plays ...c5 with a bind at d4, there will follow ...exd4, N or Q or R recaptures at d4 and White has a 'healthy' pawn majority on the K-side, the d-file is open to both players, and on the Q-side the 3 White pawns can hold on their own the 4 Black pawns. In conlcusion, if we reach a pawn endgame White will most probably win!
Now Black's trump: He/she has the Bishop pair and if the position opens up a bit he/she can use it to create tactical threats. Moreover, the endgame can be held if Black exchanges the dark-squared Bishop for a Knight and we have opposite-coloured Bishops.
Now White's sophisticated strategy is as follows: Black will try to upset the issue so that we won't reach an endgame, but in doing so he/she may upset things too much and create weaknesses around the King, so what if I don't hasten to reach the endgame and I keep the Queens on in order to attack these weaknesses? And so forth, both armies keep on refining their strategies based on the 2 fundamental imbalances:
- Black's spoiled pawn structure; and
- White missing the light-squared Bishop

msiipola wrote on 01/09/10 at 13:35:54:

You should try to learn about it instead of avoiding it. That is the key to improvement. Btw the question when exactly it is favourable to exchange and when not is something even top-GM's don't totally agree on.
On your level this is not the most important subject though. I already know what Markovich will say (and I largely agree): study tactics and strive for open positions, eg by playing 1.e4 e5 2.d4 exd4 3.c3 as White.

Yes I know. I have very little opening knowledge, but on my current level (<1500) I don't want read too much opening theory.

During the years I have spent many hours on trying to memorize opening moves, without understand the reason for those. Most of this time have been totally wasted.

I have both "Ideas behind the behind the chess openings" and FCO. But I must confess, most of the text is over my head.

The only practical use I have from those books, and other opening manuals, is to see what the first 5-6 "correct" moves are.

So going back to my question, if I don't understand why is it better to exchange a bishop for a knight, I should not do it?

In several openings you exchange the queens knight for your kings bishop. The opponent (often) gets a double pawn on the c-file, but the other player lose the bishop pair.

As beginner I have always felt such moves are bad.

But those moves are often one of the main lines, so I understand I'm wrong.

But should I, despite that, play moves which I feel is bad, even if the theory says something else?