It doesn't make a lot of sense to say a team has too many prospects, but we're also just rarely in a position to say it. Most of the players a team drafts are drafted before they are prospects, and nearly all of them are drafted before the outlines of their prospect-dom have gotten thick enough that we can tell they're crossing some other prospect's.
If the Cardinals bought all their prospects at retail prices, though, in their early 20s, this is something we would say all the time. That doesn't mean it would ever be right—it's just that we'd look at the amount of money they have, and the number of holes they have to fill on Memphis's roster, and we'd say, are you sure you guys need that second defensively suspect shortstop who has the bat to stick at multiple infield positions?
Greg Garcia is the easy comparison—he's the one we should probably start with, too, since the numbers we have from him are so much easier to understand.
He's left-handed. That would be an advantage on a lot of major league rosters, but probably not this one, where the third baseman and the second baseman are also left-handed.
Every year in full-season ball he's had an OBP over .375 and a slugging percentage under .425. He was an above-average hitter last year in the Pacific Coast League, but he managed that only after hitting .368/.471/.529 in August, which means that he spent June, July, and August falling off prospect lists.
ZiPS and Oliver think he'll keep drawing walks, but that his BAbip will fall off, his strikeout rate will spike, and his OPS will hover around .650 or .660. That is 100 points higher than Pete Kozma's, and about even with the weirdly slugging-heavy .656 Daniel Descalso put up last year.
He is 24, but nearly baseball-25—he's an August '89 birth.
Aledmys Diaz is right-handed. That would be a disadvantage on a lot of rosters, but a lot of rosters haven't tried for three years to wedge Daniel Descalso into places he didn't belong; here it works.
He has not played organized baseball in a year, and when he did it was not in something we would recognize as full-season baseball, but he did hit very well for a guy who could have been in America using aluminum bats.
He is 23, but nearly baseball-24—he's an August '90 birth (apparently.)
The best thing we can say for Aledmys Diaz from our computers is that ZiPS and Oliver have not projected his OPS to hover around .650 or .660. They have not had two years to examine his weaknesses and confirm that he can't really hit the ball hard enough to maintain that .370 OBP.
The worst thing we can say about Aledmys Diaz from our computers is that we don't know, yet—have no way of knowing—whether he's worth a four-year major league contract, and whether he'll replace Jhonny Peralta or Daniel Descalso or Greg Garcia.
I'm fine with the signing because I believe that John Mozeliak also sometimes thinks, "Do we have too many prospects?", even though he knows how ridiculous that sounds.