As far as the search for compilers is concerned, I think what is considered powerful notation should be kept around, even if it's tough to compile at the moment.
Suppose the Lisp is bootstrapped in some other language, like C or assembler. The special operators in the interpreter are written in C. If you write the IF operator in C, and that operator itself needs an if operator, it uses the C if statement or ternary operator. (Obvious, right? No level confusion.)
If you add FEXPRS, they are interpreted code themselves: interpreted code controlling the interpretation of code. If you write an IF FEXPR and it needs an if operator, and you use IF, then you get infinite regress/recursion: while trying to interpret IF, the IF FEXPR calls itself, and then runs into the same situation, calling itself again, ...
If the Lisp has a compiler and macros, then you can write an IF macro, and compile that FEXPR. Then, when the interpreter evaluates an IF form, it now dispatches a compiled function. When that function needs IF, it's just running the compiled code, and not recursing any more; the IF FEXPR is only for interpreted code.
FEXPR's can do some "impossible things", and if you want to do those things fast, compiled FEXPR's could be useful.