Important to consider pace when discussing spin.
A blooper topspin shot landing at the service line is much less effective than if the same shot lands a foot from the baseline. Same with a soft slice for that matter.
But as you add pace (hit heavier) you are able to hit with more margins for a couple reasons. 1) The ball will push through the court and 2) The spin causes the felt to get slowed via air friction and will not travel as far. When 1 and 2 are combined with pace, the opponent is forced to play deeper. Pretty simple concepts.
It's not that landing balls within 3 feet of the baseline is bad, quite the contrary. It's just really fricken hard to completely control pace and spin consistently. Even for top 10 pros.