You can't build a team based on acquiring and paying one more starter than you need 'just in case of injury'. The salary cap and market forces don't make this feasible.
The mantra of the NFL is 'next man up' for a reason. You have to develop depth at the right price, not buy more starters who are ultimately overpriced backups.
This is also my reply to all those threads saying we don't have enough WR 'because of possible injuries'.