There are Tablets like the iPad 4 with 4G with built-in 4G adapters.
With tablets like the iPad, Blackberry Playbook, and Tablets wihtout full size USB ports a dongle may not be possible.
You also may get a better deal on the tablet's hardware with your local cell phone company which subsidize the price when you sign up for a multi-year contract.
Buying a 4G or 3G battery powered Hotspot modem wich converts 3G and 4G to Wi-Fi may be better for Tablets which areWi-Fi only.