I watched a documentary once about the dangers of drinking too much aspartame, as I was drinking loads of diet coke - turns out a dangerous level would be 12 cans each and every day, which is excessive.
I drink it, and i've never had a problem.
The same sweetners are found in Muller Lights, squash, sugar free Jelly (and many, many other diet products), and nobody seems to question having those!
It is true there is no nutritional value to diet coke, but there are also next to no calories, which is why it fits with SW principles.