Currently limit_req_zone and limit_conn_zone require exactly one variable to be set.
It would be more flexible if this could be a string, similar to how the proxy_cache_key directive works, for example.

Example uses:

# restrict an end user from constantly refreshing the same page
limit_req_zone $binary_remote_addr$uri zone=ipUri:2m rate=1r/s;

# limit total number of connections to a resource (can probably already be done with a dummy variable, but looks nicer with a string)
limit_conn_zone - zone=connLimit:2m;

Yay, included in 1.7.6! http://nginx.org/en/CHANGES