it's the same reason dual wielding weapons can't block - there needs to be some trade-off for beefing up your offensive capacity. I mean I know you feel like your guy should be the most badass dude around and just have double the offense (with a weapon and a destructo, or what have you) with no penalty to your defense ability, but that would totally skew any semblance of Off/Def balance
Tbh, the offense you get is nowhere near dualwielding weapons, not enough to warrant not beeing able to block anyway imo.
Frankly what i do is if they get in melee range is either equipt another sword,just de-equipt whatever spell i was using or just hotkey a shield but i'm not made out of hotkeys and have a crapload of spells

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Hoping for a mod that will let you block aslong as you have a weapon out, or atleast a mod that adds a modifier key that you can press along whith whatever you have hotkeyd to unequipt it instead of making you dual wield it.
This is a really big oversight on the part of Bethesda then. Kinda really really bad, the PC UI has been an absolute disaster. Must be nice I guess to have that functionality.
It does, right click whatever is in your left hand in the favorites menu