[Nuttyman54]

If you've ever tried to push a really heavy shopping cart, you may have some experience with the issue. If you've got some weights, load up your frame to 150lbs, put it on carpet and try pushing it around from the rear where your drive wheels are. You'll see what we're talking about.

The casters will not always be aligned with the direction you want to move next. They may not be aligned in the same direction as each other. The swiveling action that wilsonmw04 refers to will cause your robot to not go straight when you try to drive forward, but start to veer off to the side instead. This makes the robot very unpredictable and hard to control.

Omnis are a great solution.