As long as there is no appreciable dirt in the joint and still a reasonable amount of grease then boot kit is the way to go. Ideally you would clean the joint completely and re-grease it on assembly.

The reason why joints fail due to torn boots is lack of grease. The grease not only lubicates the joint but actually is what keeps the joint cool. Most failures are due to overheated joints in the situation where the boot is torn.