We actually have some precedence for spells that are not resisted and how they are handled. Take a look at the Magic Resistance (Core, p.76) and Arcane Arrester (Run Faster, p. 111) qualities. Magic Resistance specifies that spells that require voluntary targets automatically fail on them, while Arcane Arrester creates a pseudo-resistance against spells not normally resisted and still reduces the effective hits.
To me, this speaks to the fact that spells which don't call for a defense (or resistance) is not because the target is voluntarily giving up their test. There was no test to begin with. Casting Levitate on a target doesn't call for a defense (It doesn't say "net hits must exceed..."), the spell specifies that it has a threshold equal to the mass divided by 200 kg (rounded up). Success tests have thresholds, opposed tests do not.
Your comment about Counterspelling struck a chord for me. I remembered being dumbstruck when a GM at a Missions game ruled that counterspelling couldn't be applied to a resistance test against a spell. At the time, I thought "surely that can't be right," but I looked it up, and he was right.
Still, I wanted to be sure, so I took another look at Counterspelling earlier, and I think you might be overestimating how much it should apply to. Spell Defense pretty explicitly only applies to spells that are directly targeting someone. It even specifies hostile spells too.
There is actually a pretty sizable list of spells that Spell Defense doesn't affect: passive detection spells, environmental manipulations, any illusion spell that doesn't target the one resisting the effect (like Invisibility or Mask). Even a lot of Active Detection spells wouldn't be affected by Spell Defense (by a strict reading) if it weren't explicitly called out in the characteristics for Active Detection spells. The specific rule that Counterspelling does apply trumps the general rule that it normally wouldn't.
Remember, Spell Defense only applies to you and people you've chosen to protect. You have to declare that you are protecting a particular target, which is an action (a Free Action, but those are still pretty limited in SR rules). You can't just use Counterspelling to disrupt any and all spells being cast.