Mobile devices have shattered traditional tracking methods. People visit your site on their phones in the morning, they check their PCs at work, and they take another glance at home on a tablet. Good luck getting meaningful data out of that with cookies.
There is a solution, however: User-Centric Tracking, which is at the heart of User Relationship Management. Now you can sort what your visitors do across devices, across visits and over time. Check out what UCT can do for you in this paper. (Length: 700 words; 3.5 minutes)