It is, but there's at least the possibility of doing it, which martials tend not to have. Especially with the revised Grapple rules for the Playtest, which remove the last bastions of non-AC targeting (Grapple and Shove) and make them simple AC-targeting abilities.So, this is an issue with considering spellcasters - not accounting for spell preparation.
In theory, a wizard can switch to a weak save, but that requires a few things:
1) Knowing which is the weaker save.
2) Having an appropriate spell that targets that save in your spellbook.
3) Having the spell prepared.
4) Having an appropriate spell slot available.
Which, honestly, is a pretty big stack of assumptions.
I think you're right that it's overrated as an issue. For my money, Wizards aren't a huge problem in combat, but rather out of it.