For the sweeteners that are essentially conclusively safe, like Splenda, the "danger" comes from the fact that it may work opposite of what you want.
A lot of people see fake sugar as a way to eat sweet foods without the consequence of empty carbs from sugar (empty if you don't use them ).
However, it tricks the body, and the body doesn't like it. Insulin balances can destabilize as the body receives a signal that something sweet has been consumed, yet no sugar is found. The body gets extremely confused, and essentially, the last thing you want is a biochemically confused body.
You can never avoid all carbs, as well.. if you did, it wouldn't last long - necessary to survive. Based on how well you handle the body requesting sweet foods, or carb-loaded foods, is how well eating or drinking fake sugar will end up working for weight loss. It's one method to consume less carbs, but you'll still need to consume a proportionate amount of carbs throughout the day so as to allow the body to keep insulin levels in check and not effect your cravings. Because that's all it is essentially, managing your cravings. It's not simply a willpower issue, as when the body wants something, it makes you feel like you need it, or at least really really really want it.