Rote learning is important for the most basic math facts that you will use over and over again. You should remember things like the following by rote learning:
multiplication tables
basic algebra formulas: slope of a line, quadratic equation formula, (a+b)^2, etc.
constants like e and pi
calculus formulas like integration by parts
These are just some examples.
I have to disagree with most of this. Multiplication is tables are okay to memorize by rote. It worked for me. Plus, a lot of practice doing arithmetic. However, I think the right way to do it is to practice counting like 2 4 6 8 10, 3 6 9 12, etc. I heard of a Russian teacher who got kids to come up with their own multiplication tables that way, and a lot of them were very successful when they grew up.
The slope of a line is very understandable and should not be learned by rote. Also, constants like e and pi are very understandable. Integration by parts is really just the product rule in reverse. Again, understandable. It's okay if some things kind of go over your head the first time you learn, but it's not desirable.
I use the quadratic formula by rote, but I also know you can get it by completing the square, which isn't very difficult, so if, for some reason, I forgot it, I could easily derive it again.
The fact that things like that are learned by rote is just an indication of the math being dumbed down for a "lower-level" audience.
Generally speaking, the more advanced you get in math the less you have to learn by rote, but yes, especially in pre-college math, rote learning still has its place.
Maybe that is the way it tends to be because things are taught badly, but not the way it should be. Actually, in advanced math, there are a lot of proofs that are just impenetrable and it's better not to go through the proof, at least as things stand. And there is just too much to learn when you get to research level stuff, so you always have to decide if it's worth your time. There are a lot of proofs that I know of that people working in the field typically won't know. It's my personal mission to try to make a lot of these proofs more understandable so that it will be possible to understand more proofs. But, as I said, it's important not to get stuck, and if you try to learn all the proofs in topology (and I mean graduate level or research level), let's say, you'll just end up getting stuck too much. I am the type of person who really wants to know why, but a lot of times, I skip proofs, and tell myself I will come back later and try to come up with my own crystal clear proof when I have time.