Interesting. I agree it's effort NOT speed. Also, the primary point needs to be maintaining safety...then whatever effort is going. My observations are along the lines of...
1. Yes, there is some real prick behavior out there & sadly road cycling seems to bring a lot of it out. I chalk it up to insecurity, but hey I'm no shrink.
2. Communication when joining (or inviting to join) a paceline is key. For instance, on a long & very challenging ride yesterday a group I was in had a good pace going and as we passed riders who were struggling alone a bit we invited them to hop on. In that case, we weren't going to change our pace to suit them and I don't think any of us expected them to pull (although they did and other than one being a little squirrelly they were ok).
3. Pull what you can. If it's 30 seconds, cool. If it's 30 minutes, cool. I may thank you for doing it, but then again, I don't expect to be thanked for every pull I make so I may not think it if when you drop back & past me in the line. (had someone yesterday angrily say, "you're welcome" to which I replied "thank you", but thought to myself, "isn't the pull what you normally do up front, are there any other affirmations you need me to give you?")
4. I continue to fail to see why sitting on in the rear is so offensive to some. I totally agree with the comments above that it is (as long as you're safe) a healthy way to observe & learn how a paceline works & to get stronger when you are one of the weaker riders in a group). On the same ride yesterday, there was a 40+ mile lead out that was probably averaging around 25mph. Besides the benefit of being moto-paced (it was an organized event, but not a race), there was a group of about 5-8 very strong riders who started rotating after about 20 miles and by about 30 were visibly annoyed that no one from the larger group of about 20+ riders behind were not coming up and taking pulls. There were so many assumptions being made that this could warrant a thread of its own, but basically I couldn't see any reason other than ego for being annoyed. In my case, this was the first 40 miles of a 115 mile ride w/ 9000+ feet of climbing...I was glad be part of the working paceline, but would have been equally glad if the pace were a gentler 21 or so and it was clear (to me at least) that I had nothing helpful to contribute other than filling in a spot in a continuous paceline for the people behind me. Basically it's this for me: I'll put in the work as much as I can given the ride specs/my fitness/etc....sometimes that is a lot of pulling, sometimes it's hanging on for life at the back...usually it's some reasonable point within that spectrum.
4.5 What I don't do on a ride where I can't/don't contribute much is take part in the sprint at the end...that I just can't be comfortable with.
5. Sometimes after a ride and experiences along these lines, I collect my thoughts and am thankful I don't take myself so seriously. Cycling is supposed to be fun.
6. One of the least enjoyable aspects of this sport I love so much is the personalities & behavior it seems to bring out in some. Bummer.