Chemical castration is not so "fast" as other castration modes, so, it gives some scope to prolong the experience for the one who is neutered.
Also, the body's resistance against the chemical agents give some scope for "fight" against the castration experience- a fight that is being lost anyway.
Nowadays chemical castration is perhaps the most common way of castration: just think of all those who have been branded " sex offender" -Often they are underage teens themselves ! and who face regular plysmograph examinations, to monitor their arrousal paterns and the depo provera injections that go with these, to make sure they do not get arroused by "inapropiate stimulli"