It's not that you NEED a fast lens (ie one with a small f-number) for low light work, it's just that they're better at working under these conditions.
For example, a lens at f2.8 will gather twice as much light as one at f4.
This lets the f2.8 lens have the same shutter speed as the f4 lens but with half the ISO - eg 400 ISO instead of 800 ISO. Or if you're using the same ISO, it would let you have the same shutter speed with only half the light.
The f2.8 lens will also give you a brighter view through the viewfinder, making it easier to frame in low light. It will also let you have a smaller depth of field at the same focal length.
BUT, the f2.8 lens will cost a lot lot more than the slower lens, so it's a case of weighing up the pros and cons.
For example, the Nikkor DX 17-55mm f2.8
costs around $1200 USD, where as the DX 18-200mm f3.5-5.6
costs around $900 USD. Most people would prefer to have the longer range of the 18-200mm and also its stabilisation, which after all, lets you shoot with shutter speeds around 3-4 times slower than normal.
And you'd save 300 bucks which could buy you a Nikkor 50mm f1.4
which gathers four times as much light as an f2.8 lens and leaves you a little change remaining. or indeed the 50mm f1.8 which is even cheaper still.
Once again though, lets hear from those 18-200mm owners - have you had any problems (or great successes) with this lens under low light?