For us, nobody gets us off more than we can each other. We do absolutely get off with others most of the time. If anything, I have a harder time of it than the Mrs, but it doesn't mean I'm not absolutely enjoying myself.
One exception for the Mrs is that guys with up-turned dicks can hit her g-spot from missionary better than I can. I can't help it that my dick is more torpedo shaped, I tend to have more girth and stamina than most. Everyone brings something different to the table and you make the most of what you have, but the thing that tends to get us off when playing is more mental than physical.
We both enjoy seeing/hearing each other having a good time with others, and afterwards, it makes us just want to fuck each other silly. So in this case, if someone happens to have a certain "gift", we're happy about it rather than there being any jealousy.
Once even the slightest jealousy is involved, I can see it getting into people's heads where they hold back because they think it might make their partner feel inferior. I actually know someone like that and they wound up going to counseling and got things ironed out.
While sex can be somewhat better or worse with others, we mainly see it as "different". If one of us has an extra good time, all the better, but we know at this point that it doesn't create any sort of want that would ever impact our relationship. If someone is in the LS because they're trying to make up for something they're not getting at home (other than variety of course), that usually isn't going to end well and can definitely affect people's ability to get off with others.