As Donald stated below even distribution of light this is only part of the picture. Directed light means directed from the bulb right through the lens down to the baseboard. With directed light you are concerned about the light taking the right direction: Into the back of the enlarger lens and through its aperture. Diffuse light acts more like a shotgun here, sending the light in all directions thereby passing enough through the lens. A condenser-set must have the right focal length to send its light through the aperture of the enlarging-lens instead of bouncing it against the mount or the bellows and the right image circle to illuminate the respective negative.
Originally Posted by Flotsam
Not quite as "good" systems and (I'd better said "less collimated") work with more diffusion, white domes, closer/bigger opal-lamps to achieve even illumination with universal condensers, less collimation, less sharp grain, less obvious scratches/dust. Leitz Focomats or my former Dunco 66IISW are examples for such in-between constructions.
The increase in contrast and apparently sharp grain (after adjusting contrast) when stepping form a Dunco with an universal condenser for 6x6 in a white mixing-box to a Durst L54 with dedicated condenser-sets for every focal length, all black inside, and a lamp quite distant from the negative stage was astonishing at least to the beginner I was at that time.
My personal solution to the question is simple: have both and use them as subject matter and technique suggest. Used enlargers are to cheap these days to restrict myself.