I like stamina training, not only does it allow your players to play for longer (especially important if you have a really good starter and a really bad backup), but it also allows the player to play better for longer.
For example, initially the player might play at 100%, but after 30 minutes on the court he might be tired and play at 80%. Training stamina means he might play 40 minutes before he is playing at 80%. Make sense?