Actually we do take as many jumpseaters as we have empty seats. As I have said before the vast majority of pilots understand any pilot on their own carrier should have priority.
While that may be true enough now and the common sense approach seems like it would be in use, this has NOT always been the case.
My significant other spent 6 years of his life commuting out of DEN, and has quite a collection of stories - and none of them very complimentary - about United and their jumpseat policies. And no, he was never rude or inconsiderate in any way, or had problems with anyone IRT jumpseat ettiquette. As a matter of fact he has always been grateful for any ride received. I can't speak for him, but here's what I do know -
UA used to only take as many jumpseaters as there were jumpseats installed in the cockpit of the plane - regardless of how many open seats there were in the back. If memory serves, that was finally changed in about 2005. Prior to that, even if the cabin was wide open and a CA insisted on taking an extra jumpseater to sit in back, the gate agents often would not allow it.
There was one time that he wasn't able to get home at all during a 48-hour break because of their crappy J/S policies. There was an outgoing UA flight with about 10 open seats in the back (the last one of the day out of DEN) - but he was unable to get a ride due to their policies. He had to stay in DEN instead after a 10-day trip instead of coming home to see his newborn baby boy.
Then there was the time the UA CA authorized it, but his authority vetoed by the gate agent! (Can you even imagine this happening 20 years ago?!!) Unreal. He (the UA CA) was good and pi$$ed off over that. I appreciated that at least he had tried - He was one of the few.
Another time I recall he was unable to get home because a UA mainline flight attendant refused to take the jumpseat up front - too uncomfortable!! No lie. I should also add that my boyfriend was operating as a UAX codeshare CA at the time for all of these lovely commuting experiences.
The only thing that finally changed things was when some of the regional carriers started denying jumpseats to the mainline guys. All of a sudden, the jumpseat issues just seemed to disappear. Imagine that! PFM!!
Too bad it has to come to that.