The actual tempo of a piece can only be as fast as the tempo at which all parts can be played, not just descending lines.
I agree, but I think you may be missing some things. When you are figuring out fingering, you are looking for the option that allows you to play the line and phrasing as well as you can. If the piece just happens to have a descending line that is difficult to play with alternating plucking fingers, and easier with a rake, one should choose to rake.
There is nothing to say that every piece of music will have ascending and descending lines that are of equal difficulty. So there may be times, when it's okay and even necessary to have more facility playing a descending line. IMHO, one should practice and use whatever methods allow one to best play the music.
As I have mentioned previously in this thread, I tend to practice primarily alternating fingers, but there are some times when the music requires other options. I also typically play downbeats with my middle finger, but sometimes I find it is best to start with the index because of some little oddity about the syncopation. It can be very disconcerting to try to invert your plucking if you don't ever practice this way. The same goes for raking.
So it's not just the speed that may be of benefit with the rake. A rake may set you up to be in a better plucking position later in the phrase, which could even give you a benefit in playing an ascending line.
Conceptually its similar to the various ways one might manipulate one's bowing to facilitate a technical line. For example you may play detached notes without changing the direction of the bow or do a lift, so the bow is moving in the right direction at a key moment at some point in the future.