I tend to stick to sub 14th level.. my highest 'normal' EoM is 8th level. I did a couple 20th level write-ups just to see how nastily broken it can get. Lets just say that a 20th level EoM mage can pretty much mess your day up.
But regarding the usefullness of summons..
That may be due to your perception skewed by 3.x summoning duration times. IMO summoning is the ultimate customizable utility spell...
Wanna fly? climb a wall? check for guards? get an object from under a pool of acid? etc...
For in combat casting... depending on how liberal your GM is... Swarms are considered one critter by RAW and can be summoned. Using the same line of slippery slope thought, if swarms are summonable, why not Skirmishes or MOBs?
Tell me you can't think a 20th level mage can't make some combat use for a MOB...
Alternatively.. EoM summons are not always right next to you either.. so you don't really *need* an obediant summons.. having the angry beast show up on the other side of the enemy works just as well.
And then consider add-on effects. I have one character, currently on sabbatacal, whose primary focus was caling shadow constructs that were boosted with various enhancements. A 'Harrier' {ECS} construct with flaming weapons and an zone of 'Slow' centered on him could be really useful... the possibilities go far beyond the simple critters in the MM series.
YMMV