If you sign FAs to try and win now while rebuilding, you'll usually do neither (see Angels, etc.).
A few reasons:
1) FA contracts tend to overpay the out years, so when you're ready to compete, you're stuck with albatross contracts.
2) Rebuildling top to bottom requires patience and commitment, if you're under pressure to win now, you trade Sanchez for two years of JT (Alfaro never mattered to me, I'd rather have Marchan). And if you're not going to trade prospects to win now, why commit to big FA contracts to players 30 and over (the bulk of FAs).
3) Psychology, if you're trying to do both, you compromise on both and do neither right because you haven't prioritized and your decision making is muddled.
Now when you have a lot of money, what you can do in the FA market is bargain hunt en masse, that is, you can throw around small bonuses and 1-2 year deals, cut players in camp, and just treat it as sunk costs - and that will help you find "diamonds in the rough" if you have good pro scouts/ML coaches.
Now I didn't mind the Harper (low AVV for long-term career), and Wheeler (reasonable price for strong arm that has shown no sign of velocity decline), b/c they should be solid 3-4 years from now. They're not 2-3 good years then 3 years of overpaying type deals.
But a decent organization should be able to produce internally guys like Segura, 2-3 WAR players at best, and who could suddenly decline at worst.