Loss of the tail (while saving the animals life) makes the animal vulnerable in the short and long term. It is less able to find a mate, capture food and the loss usually makes them run more slowly and unable to swim. The tail is a status signal for mates, serves as a 5th leg (no dirty jokes here) and sometimes even allows a lizard to hang suspended. The tail also serves as a balance beam while running. The tail holds stored fat and protein; which is lost to future growth and/or reproduction.
Lizards without a tail may not even be capable of physically (or physiologically) reproducing. The may not be able to attract a mate, they may not be able to do the actual act of mating due to loss of balance and they may have reduced gamete production due to low energy reserves.
A small aside from the /msg box ..."when I hear 'mating' I normally think the act, not the process, ya know?" Thanks to Cletus the Foetus for help in clarifying physical vs. physiological as it relates to lizard sex life. :D
Tail loss also makes the lizard more likely to fall prey to predators, (especially visually oriented snakes and birds) in the future. Skinks without tails are the first to bolt and therefore first seen by the predator. They are also about 15% slower than intact skinks. A skink takes about 60 days to regrow its tail. It has no safety net in case of attack during that time so it hides more and therefore captures less food and is less able to heal, bad cycle.
Faster types of lizards seem to lose their tails more often than slower lizards. Probably the slower lizard would just get eaten anyway so evolution didn't risk autotomy. The quicker (and generally smaller) the lizard the less of the tail that will be shed; it may be just the tip. A slower, tail shedding lizard (often also a larger lizard) will lose almost the entire tail. (The better to feed you with and keep you occupied longer - my dear predator!). Many lizards have brighter colored tails when young (and smaller) to attract predators to this *relatively* more expendable piece of the body and away from the head and torso.