In solo play it is a tiny bit faster to keep an unnamed portal at your home or hub for emergencies. That way you don't have to take the extra time to name the portal in an emergency. Of course this doesn't work so well in group play.

In re protecting portals, there are two times an unprotected portal is vulnerable - one when you dive through with a mob that has an AoE in close pursuit. The other as you are coming through. There is a brief period while you are still 'in transit' but the destination is loaded and active. I have had a portal collapse just after I stepped through.

[edit] Three times.. an unprotected portal is vulnerable as long as you are close enough for the zone containing it to be active.

PS: there have been, over time, some updates that have affected this situation.