Is there any reason other than potential confusion when one of the two backquotes is accidentally omitted?
I thought about this a while ago and I think it probably simply has to do with complexity of expressions. If you allow arbitrary expressions to appear within the ticks, you have a problem with: x `f a `b` g c` y does this mean (b (f a) (g c)) x y or f a x (g c b y) or what? You could impose the constraint that you can't have embedded ticks, but this would grossify the CFG. Furthermore, you then have the case of, why isn't this valid: a `f (x `g` y)` b where the embedding is unambiguous because of the parentheses. i don't really know, but i find this fairly difficult to read. a `h` b where h = x `g` y is a lot simpler imo... - Hal