These contracts are much more likely to be used on draft day or the trade deadline than they are next July. C's should be well under the cap with or without them.
1) On draft day they will be tradable, expiring contracts, so that a team with a star Danny wants can make the trade then and not after the moratorium (since, as we've witnessed, nothing is guaranteed that period).
2) At the trade deadline they are like expiring contracts for matching purposes, but ones the team we're trading with can ship off on draft day.
Their use next summer is limited (and we don't know how long they're unguaranteed for). Their best use is for one of the 2-3 teams that will start next summer over the cap. For example, Cleveland will be above the cap. Say they wanted to acquire, via trade, a player or players who make(s) $17 million. The only expiring salary they have next year is Varejao, at $9.3 million. AV can only bring back $14 million in salary. However, if they traded him to the Celtics (or the Sixers if that made more sense), they could acquire Amir's $12 million contract. They could then turn around and trade Amir (by himself) for $17 million in salaries, thus leveraging the value of Vareajo's $9.3 million. Alternatively, Cleveland could trade it for a $12 million exception to be used later in the year. OKC could do the same with Anthony Morrow's non-guaranteed contract, and gain a larger matching tool by swapping it for Jerebko's. Or, if a team had $5 million in cap room, they could acquire Jerebko's contract and immediately turn around and use it with other players to increase the amount the can take back in a trade that left them over the cap (since you don't have any restrictions on trading newly acquired players if you traded for them with cap room).