purchases and drops

I've been playing Mass Effect again, after re-buying it through Games on Demand (partially to test the service), and I noticed something common in games that annoys me. I can purchase items, but the same items are commonly dropped as loot. Half the time I buy something, I find that same item not long after. In effect, I'm paying for early access, rather than actual ownership.

Should loot and purchasable items be the same? It's a question often asked in regard to MMOs, but it should be considered for other games as well. Actually, the MMO question is usually "Which should be better?", a follow-up to the one I pose.

How might a player act if he knows purchasable items appear as drops, too?

On the one hand, the player might hoard his money, as I do. He might ignore the stores almost entirely until he can afford those few epic store items that don't drop. In this case, the store is almost pointless to include (assuming the game doesn't allow the player to lose items by death or failure).

On the other hand, the player might spend freely. Now, the store is fun, but the loot loses its value since the player usually has what he wants already when something drops. Or the player is disappointed whenever he finds he has "wasted" money on something that drops shortly after.

It makes sense to me to make purchasable items and looted items different, because they seem to interfere with each other otherwise.

One way to do this is to divide sale items and loot categorically (ex: armor is looted, but armor augments and customizations must be purchased). But I'm sure there are many possible avenues.