At first, I thought Danny was nuts with this pick. As I looked up Bradley, I figured he can probably be a good bench combo guard/insurance policy. I think he can probably back up both guard spots off the bench for about 15-20 minutes total, quite likely even this coming year. Though not a position of need (so we thought), I realized the genius of Danny Ainge.
Most of us agree that Nate and TA are both career backups for various reasons, so there is a clear limit on how much a team can afford to give a bench player. Perhaps Nate and TA have proven themselves to be worth upwards of $3 million/year by showing good in the playoffs when it counts, and possibly for 3+ years. No worries any more, as Bradley could replace at least one, and perhaps both of them in the rotation (assuming a SF FA signing or trade).
Short take: He might replace Nate, Tony, or both, but the pick helped the C's negotiating position considerably when talks w/agents come down to numbers. Danny won't be held up by Nate and/or TA because they have limited ability to add players otherwise with too many needs to fill.