While I wouldn't describe the trade exception as "insanely valuable" like Forsberg does, it does have a lot of value. We've seen teams like Cleveland and Utah gain assets for absorbing other teams' unwanted salary, and there's a chance that this can be used.
The value is mitigated to some degree because at least for this season, the team is unlikely to want to add salary. Come draft time and early next summer, though, Danny could use this to add more to the deal.
I didn't like the trade for two reasons: 1) I thought the Celtics should reload, and 2) I didn't think the Celtics got good value. The trade exception could potentially help a lot with #2.