Well, if you're riding a frame size/design which offers plenty of clearance to allow easy access to bottles you wouldn't. Why deviate from the lighter, more proven design of the Custom Race? For one, the Custom Race isn't very friendly if you've got tight bottle clearances. The Cannibal is likely to be marketed at those of us riding smaller and/or sloping frames. If you struggle with removing your bottle because your top tube or head tube is in the way then these cages are a dream come true.
Unlike traditional front loading cages you can load your bottle from the side, and unlike traditional side loading bottle cages you are not restricted to which side you can load from. Elite's design allows easy loading from any direction, left, right, and front. This makes it a very versatile cage, especially if you're ambidextrous in terms of bottle grabbing. Many of us may have preferences as to which hand/side they prefer to use to grab their bottle, and certainly after becoming accustomed to using a traditional side loading cage you become used to only using one side. However, being able to use both sides gives the rider more freedom, your bottle cage is no longer dictating which hand you must use to grab your bottle. It may sound gimmicky, but it's surprisingly handy in some situations.
What really sets this cage out from many other cages (particularly many side loaders) is that bottle entry/exit is swift. Traditionally, with many side loaders it was a two part action, involving sliding your bottle in at an angle, and then straightening. With the Elite Cannibal cage no finesse is required, simply force the bottle into the cage, and the Cannibal will consume the bottle.