I wouldn't worry too much about the cost of parsing. I'd be a lot more concerned about APIs that rely on proxy and UA caches for performance and/or scaling. A

I wouldn't worry too much about the cost of parsing. I'd be a lot more concerned about APIs that rely on proxy and UA caches for performance and/or scaling. A request has a much better chance of getting cached reliably if all the important semantics are in the path than if they're in arbitrarily-ordered querystring args or headers. Squid's default config didn't cache anything with a querystring up through v3. OldIE wouldn't cache an uncompressed response with any Vary headers. I suspect there are landmines like this buried all over the place.