Avoiding capturing is one of the keys to writing quicker regexes. Using (?:...) rather than (...) when you don't need the capture can make a substantial difference to the speed at which they run, especially if the regex does a lot of backtracking; which is another thing to avoid where possible.
So using zero-length assertions instead of captures probably has the same benefits. It also makes the purpose of the regex less obscure IMO.
Note: This is just based my personal experiences of using the re, rather than any insight into the workings of the re.
Examine what is said, not who speaks.
"Efficiency is intelligent laziness." -David Dunham
"Think for yourself!" - Abigail
Hooray!