When David Robinson went down, they went down and grabbed Tim Duncan.
When Tim Duncan went down, they got knocked off from playoff easily. But still, you go for it when you see an opportunity.

There's no guarrantee that Al Jefferson won't get hurt (c.f. Amare Staoudmire/Antonio McDyess) or will live up to his hype, and there's no guarrantee that Allen/Pierce will be around when Jefferson matures.