In "The Mythical Man-Month", Fred Brooks writes about two concepts that I think are important considerations when deciding whether to leave the experienced programmer alone or train up the team:

* The communication overhead that adding more programmers adds. Training time can be thought of as a manifestation of this tradeoff. * The conceptual cohesiveness that comes from having one developer or a small team design a system, as opposed to a free-for-all design that can arise when a large team is unleashed on a program.

Whether or not to train teammates is (ideally) a development strategy, given the needs of the business.