The lesson of this problem is pretty simple: don’t make blocking points against a back game unless there’s some chance of actually blocking your opponent. The obvious play is 13/8, filling in the last blocking point in Black’s prime. The blot left on the 13-point is of no consequence in view of White’s weak board. However, Black has no need for the 8-point. White has plenty of time to maneuver and build his board with the four spare checkers on his 13-point and 8-point, plus the spare on the 20-point which can be released with any five or six. If he makes the 8-point now, Black will find himself needing to clear it in a few rolls, at a time when White’s board is much stronger than it is now.
Instead, Black has a chance to address the real problem in his position – his open 3-point. It’s true that Black has several builders bearing on the 3-point, and each turn he’s a little less than even money to make the point naturally. But it’s a disaster for him if he never makes the point – then White is in effect playing a 2-3-5 back game, and Black will be leaving plenty of shots when White’s board may be strong enough to win.
The right play is to go ahead and slot now with 8/3! Not only is White unlikely to hit, but most of his possible hits are too dangerous for him in light of his weak board. If he throws a hitting number, he’ll mostly be better off passing on the hit and just building his position. In effect, the 3-2 roll gives Black a free shot to build the most critical point available.