1) You are still supporting some of the most awful companies on the planet if you drink their 'diet versions' of the same stuff that an obese 10 year old is spending 75 cents at 7-11 to get a portion which would exceed two adults t-dawg 2.0 carb requirements for an entire day - and that is for his drink on the way home from school. Same companies that want to exchange funding for product placement within the schools. Basically, the same products that play a big role in why as a nation, we are more fat than in shape, and have a nauseating number of diabetics running around. Do you want to support that company?
2)You are also cultivating more of a sweet tooth than you probably want. Since studies show that tastes are indeed learned and alterable, why give yourself further desire for sweets? Maybe if you took 2 weeks and didn't drink that stuff, you would crave it less and crave something else more.
3) Artificial Sweeteners are processed in a manner that the product you get includes tiny amounts of heavy metals residue. You don't want those inside your body. I'm sure that we are all exposed to them plenty as it is, so getting more of them might not be such a hot idea, especially considering that they tend to stay in the body. However, something like grapefruit pectin has been shown to bind to heavy metals and help 'cleanse' the body from them. Will this appear in your physique? Not really.. not likely at all.. but with all the increased instances of alzheimers and other problems being linked to heavy metals (aluminum chlorohydrate, etc.),and considering a bottle of grapefruit pectin goes a long way for being roughly the same cost as a big bottle of Vitamin C, I'd say to just take that little precaution.
4) If you crave flavor, their are flavored waters that are no where near as bad for your health, despite being artificially flavored (anyone who has read 'fast food nation' knows that those 'artificial flavors' are some complex chemistry)
5) Some people are sensitive to the artificial sweeteners in a very concrete manner - thats an obvious one to consider - if it makes you feel shitty or weird, you shouldn't be drinking it.
6) I don't want to think about where their water supplies come from..
Make up your own mind, but definitely, please keep the first point in mind. Insurance is expensive enough as it is, and supporting these companies is not helping the situation.