We just returned from our first trip to Vegas. DH had a conference, so our rental car was paid for by his work. But even if it wasn't, I would have wanted the car. No, we didn't use it every single day, some days we just walked the strip. (we were there for a week) But I loved having it to easily get to places off the strip (Target for some sodas/snacks for the room, the Outlet malls, various restaurants, plus a side trip to Red Rock Canyon, and also to the Hoover Dam).
I also rent a car at WDW each time, because I just can't be bothered with the hassle of waiting around for the buses and such. i guess i am just a go where i want to go when i want to go person.
If you are only going to be there for a few days & have no interest in doing anything off of the strip, then you may not really find a car useful. just compare the cost of a car to the cost of taxi to/from the airport & see what works best. Like other poster said, there are LONG lines at each casino entrance to catch a taxi, so be prepared to wait.