NO you really do not need any external drirves with that new super speed SSD. I am not a mac person but I have done testing on my experimental desktop and my PC benchmark shows that with the 3000 MHz class of storage the old hard disk rules of 4 to 5 disks are obsolete. With 10 to 20 times the performance of these new devices over hard disk drives Premiere Pro works fine at least in the PC environment. Just make sure you do not have all sorts of processes running while editing.
If you do decide that you want one external, such as for backups, consider a dock and a bare drive
A possibility is this USB3 dock with fan to swap drives http://www.amazon.com/StarTech-SuperSpeed-Docking-Station-Cooling/dp/B0055PL2YI or http://www.newegg.com/Product/Product.aspx?Item=N82E16820147511
I use both those, I have a Star Tech for using my occasional old hard drive archive. Also I have fallen in love with the Samsung T series portable USB SSD's. I have one of each T1, T3, and now a T5.