From my understanding the main benefit of RE2 is not speed, but linearly scaling execution time with respect to the input size, along with bounded memory usage. For certain inputs, it may outperform other engines, but the converse may also be true. As with any feature that may be abused, backtracking may also be useful: for example, you may need to write a script to munge text. Since you're not exposing it to arbitrary user input, it's a reasonable feature.
RE2 is not dramatically faster than all other regex implementations, but it is dramatically faster than boost::regex, which is among the slowest I've ever tested.
