You don't need a rental car in Vegas. That said, I almost always get one.
Getting a car opens up a world of off strip opportunities: great restaurants, shopping, gambling and nature. Yes, there are restaurants, shopping and gambling on the strip, but the folks on the strip know they have a captive audience and don't always provide the best service and pricing. Sometimes it's nice to get away from all that. And if you want to get out and see some nature - Red Rock Canyon, Mt Charleston, etc. a car is really the way to go.
Cab between the airport and strip resorts will run you about $15 with tip. Cabs between resorts on the strip are about $10. Unless you are staying at one of the resorts with a monorail stop, the monorail isn't a good transportation option. You have to take long walks through the casino to get to the station. The Deuce bus is $5 a day and does make for good cheap transportation up and down the strip.