https://www.mylookout.com/ - I have used this in the past and liked its features. It provides you the ability to backup contacts, text messages, pictures, and such as well as provide a virus check on every app that gets installed. Also if you lose your phone it has a service via the internet to locate it or send a loud ring to it so you can find out. And this is all with the free version. The paid version gives you the ability to wipe all of your data if you have lost your phone or it was stolen. Speed wise you might lose a few seconds during an install while it scans the app for viruses, other than that I liked it overall. I would hope you wouldn't need a virus scan app if you are installing from the Market only and not from 3rd party apps.
You really don't need one, however, peace of mind guarantee helps. I use Lookout, the paid version. It not only scans for viruses, it has backup capabilities, and a "find my phone" function. The nice thing about this apps is that it doesn't get in your way like some viruses scanners do. Norton also has a beta version for mobile phones, however, this one might prove to be pricey as all Norton products are.
I'm using netqin anti-virus and I like it better than lookout, it has the same features as lookout has but it scans twice as fast. I'm also using android booster, which I am liking because it has a toggle to turn off the 3G, I haven't found a virus yet while using these programs but its better safe than sorry.