There is never a reason to use them, though. They can be replaced by the shell's && and || operators, which nicely bypasses the parsing ambiguities at the same time.
Not sure I agree. The equivalent of more complicated expressions like (a AND b) OR (c AND d) with && and || isn't always so "nice" IMO. e.g., try it for this:
[ -f a -a -f b -o -f c -a -f d ]
{ test -f a && test -f b; } || { test -f c && test -f d; }
Yes, it's more verbose, but the original expression isn't particularly readable either, and if you'd want to evaluate something like (a OR b) AND (c OR d) you'd need to introduce parentheses anyway.
([ -f a ] && [ -f b ]) || ([ -f c ] && [ -f d ])
(test -f a && test -f b) || (test -f c && test -f d){[ -f a ] && [ -f b ] ;} || { [ -f c ] && [ -f d ] ;}
And yes I know it creates subshells, that was precisely what I was saying in my comment where I said the simplest syntax I could think of (i.e. that very code) creates subshells.