Nick wroteCOLONWho Said long term? Missing 3gp (one week) happens regularly.
It makes absolutely no sense to quantify the merits of one method over another on a short term basis, but I will play along for the sake of discussion:
Just like a starter can go down with a tandem, a starter can go down in front of a quality back up on another team. Allowing you to pick up more starts, and quality ones at that. The potential also exists for a quality back-up to get hot, or supplant the starter entirely.
Alot of things can happen under a "what if" scenario, which is precisely why you don't make decisions based on them. You make decisions based on the assumption that everyone will stay healthy, and perform at or near their career rates.