According to FireEye, the average enterprise organization is attacked by malware once every three minutes, with each attack costing $3,000 per day or more to recover. Yikes. Opening up the corporate network to rogue BYOD devices increases the likelihood of these costly attacks.
There are security tools and policies that can be enabled on a personal device that allow it to play nicely in a corporate environment and minimize risk, but with BYOD, that risk still will exist. The risk, however, can be controlled, and the associated competitive risk of not mobilizing is far greater.