Cap space isn't that necessary for restricted free agents. For a restricted free agent, who is made a qualifying offer, to change teams, it means one of two things has happened:
1) The new team made such a large offer that the original team decided not too match it. You've probably significantly overpaid.
2) The new team and the original team came to an agreement about a sign-and-trade. In this case, you're giving something up, but you might not be overpaying by as much, and cap space isn't requisite. The cost might not be that high if you're acquiring the player with a trade exception, such that the old team doesn't have to take on any salary.
Accordingly, clearing up cap space to make a go at restricted free agents is generally not useful. If there's a player we really want, he can be acquired in a sign-and-trade. Otherwise, the salary would probably be significantly higher.