It is the only realistic option for ISIS at this point - everyone else is closing in to kill them.
I don't know why ISIS has not gone along with their (covert) Turkish patrons (driving a hard bargain perhaps), but if they do, al Bab could fall suddenly. Such negotiations could explain the different reports from high Turkish officials - it might not be so much about the fight on the ground, as it is about a secret deal to rig it.
If ISIS determines to fight at all costs (many in the al Bab garrison are local natives), it could potentially impact the big Turkish referendum in April, where Erdogan is trying to accumulate new powers.
Kurdish success will have to come through military success, whereas the Turks can succeed through political dealing, because they have a lot of military, economic and strategic chips to trade.
Rumors on twitter are now saying that the Kurds have cut the main road East from Raqqa to Deir ez Zour (which I anticipated). If true, Raqqa would be cut off from vehicle traffic, since the bridges are now down. There is a long urban strip along the river between that cut in the road and the city, which would take a lot of effort to clear, if ISIS fights for it.
Hopefully, ISIS will have to pull back into the city soon, allowing the Kurds to consolidate the rural areas, and bring in the cordon for a deliberate assault on the city. If the Kurds have a tight cordon, and are effectively assaulting the city (in say,the next month), then there would be much less interest in letting the Turks take that mission. So there is a bit of a race on to take Raqqa, between the Turks and Kurds - the next two months (until the Turkish referendum) are pivotal.
