I have absolutely no problems with someone joining our/my paceline given 2 things:

1. You can ride safely in the paceline and know to signal, not overlap, etc. etc. etc.
2. If you take a turn at the front you maintain the same effort.

Given those 2 rules, if you are getting to the front and slowing the group down because you feel that is how you can contribute you are actually breaking both. IMO you are far better off taking a short pull at the appropriate effort level OR staying at the very back. A trick I've used in the past on days where I don't have the legs is to swap onto the wheel of the person falling back (from the back of the paceline) so when they check to see where the back of the line is to fall back in, they don't see me in line and naturally fall in line ahead of me. I just move back with them and repeat with the next guy.