If you think it's normal to write sin(x), cos(x), or log(x), you've been away from math for too long. Those would nearly always be written much more conveniently as sin x, cos x, and log x. You see parentheses when you have complicated arguments, but using parens in sin(3x+4) doesn't really differ conceptually from using them in 5 · (3x+4), and nobody thinks you need parens for multiplication -- compare 5 · 3x or just a simple statement like 15 · 4 = 60.

f(x) is different; you're right about that one.