There are many other benefits to co-teaching.  Even those who have no plans to train, to disciple, or to build up the other teacher will find that co-teaching can lead to great improvement in the quality of their teaching ministry.
The dynamics of brainstorming with another person while planning a lesson will result in lessons that are more poignant and better organized.  The two teachers can explore ways to make the lesson more learner-friendly and then, after the lesson is taught, they can evaluate its effectiveness.  If you are a teacher who does extensive lesson planning, then working with a "brainstorm-buddy" during the planning will allow you to build better lessons in much less time.

People working solo can become sluggish and discouraged.  They can get to the point where they are merely "going through the motions."  Working with a partner provides "instant momentum" – jumpstarting your activities and thereby overcoming these common problems.

Co-teaching can also be better for the students.  Having two (or more) teachers deliver the lesson can help overcome attention span problems and allow for the targeting of more than one objective (with each teacher targeting a different one).