XML, JSON, Protobuf can all be used as object serialization formats. It's not nonsensical at all for an endpoint to offer choice as to what serialization format a client may want to use. It's not common, but that doesn't mean nonsensical.
URLs are much longer, harder to remember and harder to communicate than accept header values. It also unnecessarily complicates the API. It also relies on out-of-band/non-standardized information to know which URL returns which format, where the accept headers is in-band/standardize information for specifying the return format. Seems like there are more multiple advantages.
