Rest strokes are very useful for fast loud scale passages. I can't imagine playing the Aranjuez without using rest strokes, and even Villa-Lobos etude no. 7 is, to my way of thinking, pretty hard to play correctly without rest strokes. Notice, however, that I said "pretty hard to play", not "impossible to play". I'm sure there will be a barrage of messages chastizing me for defending an outdated technique, and naming any number of major players who execute it flawlessly using nothing but free strokes. In fact, Barrueco has claimed that he never uses rest strokes. (He said it once directly to me, in 1976. I don't know if he would still make that claim.)
I use rest strokes sparingly because they tend to be loud, percussive, and if over-used can just make the music sound heavy handed. But for the fast scale passages that I mentioned above, and others that are similar I use them. That said, it is very important that you learn to execute both free and rest strokes correctly. It may be a bit easier to play the free stroke incorrectly, so make sure that you are planting the fingers correctly, pushing from the knuckles, and following through correctly.