I'm surprised that the most recent information/clarification hasn't even been mentioned in this thread. Here:
RPG Site: Primal fights will become roaming in 2.0 for the summoning system; are there any specifics you could go into? Will weapons and things like that still be obtainable, and will there still be instanced fights like which are available in the current version?
Yoshida: Most of the primals will not be “roaming” the public areas, and their battles will still be instanced, though it will be more challenging to discover these instances.
Koji: It looks like there was a little bit of misinformation given at the E3 interview (and this may have been my fault, or it may have been Yoshida-san’s). The implication given was that all the primals would be roaming, and once they were summoned, they would pop somewhere else in the world. This is not the case. While roaming primal are still not out of the question, the summonable deities will be instanced-based, with the primal going back to these instances once they’ve been summoned. However, players will still have to look for these instances once they’ve become available, preventing camping.
Source: http://www.rpgsite.net/articles/397-...oper-interview


Clarifies that the primals will not actually be roaming, and the fights will still be instanced, with the instances changing location every time the primal is summoned.

Also note that Yoshi says MOST of the primals will not be roaming. We already know, for instance, that Odin will be a roaming primal.