I think the argument that you need many cities to slot the great works is overrated - the culture victory is won through multipliers and theming bonuses, and every time I won the CV so far, my capital (or, more rarely, another designated GW city) was generating over 90% of my tourism.
I believe museum is the only general building which allows you to store more than one work of art (and thus offers a theming bonus) - everything else comes from world wonders and national wonders.
The only reason why you would want more cities than one is to send food caravans to the city or cities that are running the GWAM specialists.
There are lots of reasons to have more than one city in a cultural game.
1. You still need science if you're going to build wonders before the AI does.
2. You still need gold to maintain all of those cultural buildings and to pay for your army.
3. You can get more luxuries, which gives you more happiness, which leads to golden ages and culture.
4. You'll have more production to put into the International Games, which is insanely good for a cultural game.
5. You'll have more artifacts, which generates more Tourism.
6. And lastly, 120 Tourism is obviously better than 100 Tourism. If you can get 18 from just a Museum, why not found a city, build a Museum, and stuff it with artifacts?