Must admit, having been around a lot of sport for a lot of years this issue doesn't really concern me. Cain's right No Doz have been around since I was at school...and that's a tad longer than 13 years
Basically you could get the same effect by drinking the equivalent in coffees..it's just a lot more convenient to pop a tablet.
Richo has also admitted taking caffeine tablets....as well as being a heavy coffe drinker. In fact many players have admitted taking them. As Everitt and Brown said they suit some players and don't suit others.
Players also take amino acids and various other products (legal) to aid in recovery and to help give a lift. Players drink sports drinks with various additives, eat lollies, take glucose tablets...the list is endless. All are legal and all are taken in the belief they'll help in some way.
I'll bet we've all drunk a heap of coffee at some stage to stay awake and to help focus...I know when I was at Uni, and completing assigments at the last minute, coffee was my drug of choice (along with nicotine).
If it's legal, has no detrimental side effects and it helps I don't see that it's a problem.