For us, we use just the one brand. While the alure of multiple brands is enticing, the thing that you must remember is that with multiple brands you must have multiple marketing campaigns - this ultimately means you're spending twice as much money to compete against yourself in the same market.
Now if you're running a budget hosing campaign in one company, and a VPS campaign in a different company - then you're in different markets, but again you have to spend twice as much money, and risk losing the lower end users who will seek out an alternet company when they want to upgrade.
If you run a single brand covering multiple markets, you can now take your twice as much budget and flood the market for a single brand.
A multi-brand strategy allows you to take more "shelf space", or in our case, search engine results space. You can saturate a market by filling all price and quality gaps, and you can cater to brand switchers who like to experiment.
For example, the Endurance Internaltional Group owns many web hosting brands - FatCow, iPower, iPage, homestead, bluehost, hostgator, a small orange, justhost and domain.com - these are all under one major organization, but by having multiple brands they can corner a market pretty easily - and take advantage of those that like to bounce between places too!
Just keep in mind that multiple brands means multiple marketing strategies.