That rule they ditched (well, maybe just ignored or forgot) had reasons to exist:

I wish both 5e and Larian adopted the fixed version of the rule:

A character can only cast one non-cantrip spell during their turn, no matter what actions they have available to them. They can combine that spell with any number of cantrips that they have the actions for. This does not restrict spells cast not during your turn, such as a Hellish Rebuke cast as a reaction.

If I still played P&P, I would adopt this rule as a DM. Wizards/Sorcs are OP even without completely ditching that rule...