Adding a big scorer off the bench makes sense, but adding the lockdown defender may help but not as much. The lockdown defender can't lock anyone specific down except perhaps in man defense, and even then you couldn't assign him to anyone - he'd just come into the game at wherever you indicated on the depth chart.
Also, even in a zone or press a better defender helps but unless he is an offensive threat too he could actually jeopardize the game for you because assuming the other team can get some buckets you may have a harder time doing so on the other end. I guess there is a trade-off there in that situation, so possibly more strategy is involved.
Another thing to consider would be the SPECIFIC defensive alignment at the end of the game. You want your best player getting the ball, but if I'm on the other side, I want to deny him the ball. Giving coaches the option to allow a certain player to take the final shot should be counterbalanced by giving other coaches an end-of-game option to play a "deny the ball" defense on, say, up to two specific players (similar to the way double-teams are assigned now, except it can be done in any defense at the end of the game).