The map is designed to force players to split up. So yes, a point never will be fully guarded at that state. But its not strictly a 3 and 3 situation. Many times a point is covered by 1 or 2 players who overlook the actions there and alert other players.
On anything below advanced there has been some ensurance that there wont be too much pressure on both points so essentialy all 5 players can just switch constantly between points. And on normal difficulty the bomb isnt active during the phase where both A and B are active.
On advanced you get the first moment where some planning on positioning could be required, but a team that learnt the map no longer would have to communicate, and experienced players who already handled expert easily also wont have to. Advanced is easy enough to give players a chance.
Only on expert both points can get some serious hazards and prioritizing the defence can be a must.
But even on expert, you will eventualy return to a state you already had in mannhattan (1 point to defend, 1 or 2 bombers, or 3 bombers at once and no points remaining).