@desolutionist I agree with your premise. I'm not sure that's the effect of this card:
This only hits the second spell. It's pretty hard to play multiple spells against shops as it is. I grant you, it would nerf opposing Moxen pretty good. Maybe that is huge... Idk.
I will say, that when I beat Shops, it's usually because I got there on mana to drop that one backbreaking spell (Be it Hurkyls or Enery Flux or something else... Oath) And when I lose, it's usually because I never got there (ARGH... one mana short. SO frustrating.) Rarely do I lose because I couldn't cast a second spell. (I don't often play Xerox though... So in that case you might be/are likely right.)
For example. If I were holding a normalish, opening Oath hand (I play Oath like 90% of the time) that included a Fetch + Force + Preordain, and an opponent on the play dropped Workshop into Damping Sphere. I would snap resolve this onto the board. It doesn't even prevent me from Forcing something else. And on my turn, I can play preordain fetching basic, and (assuming there is another blue card somewhere) I can still have Force up again on their turn. If I need to drop Oath or Hurk, this doesn't stop me from doing that. Heck, if it's Hurk, then I can play a spell on my turn, then Hurk them on their turn and Damping Sphere does nothing.
Again - I totally agree with your statement that turning off Workshops extra mana is worth it for an extra sphere. I just don't think that this thing really counts as an extra sphere in most cases.