You only require CPL knowledge to become an instructor - you don't actually need a licence issued. If you intend being a captain on the big stuff, then the CPL/IR is useless, but there are plenty of people with CPLs enjoying themselves with good jobs. You can fly as captain on reasonably large machines with one, but that's not the issue. I remember parking up next to a long row of DC 10s in Canada (shows my age) in my DHC2 (Beaver) and there were a lot of aircrew who came to visit and said they wished they could swap places.