When a top-pairing defenseman becomes available at that price (which is almost never), with a very fair/affordable cap hit, you have to be willing to pull the trigger quickly. Maybe you pass up the opportunity to milk every drop of value from the trade but you can't afford to lose out on the deal.
When people watch Schmidt play, day-in-and-day-out, there won't be a single criticism of this trade. This is a guy who literally tilts the ice almost every time he's on (plus-129 shot differential last season), while playing his team's toughest minutes. Extra assets be damned.