Why indeed. Some fantastic renders can be done in Blender Internal. Cycles is the future of Blender. It can produce a more realistic material. Glass and translucent skin are good examples. Cycles more closely imitates light. A pin hole camera can be simulated in cycles. Depending on the style of render you are trying to achieve BI may be the better faster choice. 😉 Once you get used to cycles and provided you have enough render power, cycles is wonderful. Here is a link that might help.
https://cgcookie.com/image/blender-internal-vs-cycles/
