Demon Portal can be very good for certain Mage builds. You usually use it so you hope to get Archdemons.
I've won a few nifty matches that way. I admit though, I hate the random nature of it and I usually prefer more deterministic spells.
Plague is the only other debuff I would use when Helplessness is not an option (due to +60 attack over defense)... since it works on Black Dragons and other Level 5s (Pygmy does not). So I can lower their HP by -15% AND their attack/defense AND it inflicts a de-buff agains their morale.
Shoals of Pirana is very situational (great on that bridge battle). A lot of those new spells were pretty "worthless".
Ice Orb was marginally useful, but mostly eh.
I agree with Glots Armor, but I also find I am usually better off with using Phantom instead. It isn't always easy to predict who they will use their imba attack on and I usually have trouble when they gang up on a single stack, so glot wouldn't really save me all that much.
I agree with your other sentiments and clarifications.