Virtually no teams are made of 100% incompetent people. If you've got a 20 person team with two worthless people on it, it's barely even noticeable from the outside.
The real problem is when you have someone who is just competent enough to deliver mostly-working bit terrible code. Which the rest of the team has to maintain.
If the person is fresh out of college, of course, this is understandable and a senior person will likely be mentoring them, and reviewing their code. But when you have two such people who gravitate to reviewing each other's code, you have a serious problem that may not be immediately apparent.
You are correct, for the most part. Even on a smaller team, if you have stronger and weaker programmers, the end result tends to be a functional product. However, I think it tends to create stress on the team, as the stronger programmers feel like they have to pick up the slack and be extra mindful for mistakes made by their counterparts.
