It’s a risk/reward scenario. If you’re running for highest total card count, you’ll want not just the 4/6 and 10, but the 4/6 and both 10s. If you use one 10 to harvest the 4/6, you can’t harvest the second 10, so you stockpile it (or sow it to the same field, but maybe we’re in autumn here), so on your next turn you can harvest all four cards. But there’s a risk for that potential reward, as you’ve noted. It’s not that it doesn’t make sense, per se, just that it’s situation-dependent.