There are features of some regular expressions for which the only known solution is backtracking. If you want those features then you "require backtracking".
EDIT: you are correct, backreferences do require backtracking, my bad.
There's no RE2-compatible regular expression for matching palindromes, but additional features as found in PCRE and similar "regex" engines can do it with backreferences or with look-around assertions. See http://stackoverflow.com/q/3746487 and http://stackoverflow.com/q/3664881 for two ways to write such a regex.