It's risk-aversion. Think of it this way: if drive-models are assigned to Macs in a round-robin fashion, then if there are three drive manufacturers and one has shipped a buggy drive, at most 1/3 of customers would be sending their laptops back for repairs. On the other hand, if their only supplier gave them buggy drives, then everyone would be sending their laptops back.
It also lets them have a stronger negotiating position to drop a crap supplier. They can only maintain that negotiating position, though, if they buy drives from all of them; if they only used drives from whoever was the at-the-time "best" supplier, then if that supplier started to suck, all the others would demand a king's ransom for the "privilege" of switching to them.
Interesting. I could also imagine that the inverse of that point is also true: if one of the suppliers is really good and reliably so, if Apple depends on them more than the others, the cash inflow will help that one supplier more than the others, which that company could put back into R&D to widen that quality gap. And if one supplier is far superior to the others, then the comparability == competition between suppliers lessens, and so does Apple's bargaining power.
It also lets them have a stronger negotiating position to drop a crap supplier. They can only maintain that negotiating position, though, if they buy drives from all of them; if they only used drives from whoever was the at-the-time "best" supplier, then if that supplier started to suck, all the others would demand a king's ransom for the "privilege" of switching to them.