You're kidding, right? Do you think the Rangers should have buried McD in the minors to save some $$ on his next contract, even though bringing up a talented kid was better in his 1st full season than any D-man we have on our team helped them win games and improve as a team?
Sure, his next contract won't be cheap, but look at what other very good young defensemen got in their 2nd contracts- Subban only got $2.875m per in cap hit, and Del Zotto (who nailed down a top 4 spot) has a cap hit of $2.555. Those are very manageable amounts for guys who help a team win. The Rangers potential cap woes don't have anything to do with playing their young guys- it's more a product of their FA and trade activities.
In the Red Wing world, KH may have kept Smith down in GR an extra season to only give him this 1 full season up with the Wings before his next deal comes up, but I don't think the potential contract savings is worth the cost to his development he could have had in playing a season with Lidstrom.