I own books with verb tables for French, German and Spanish, i.e. for several languages with rather elaborate verb conjugations. At school, I was made to learn such conjugations for French verbs and quizzed on them. For Dutch, Latin, English and German, we basically learnt the principal parts and learnt to derive the rest from these.
However, are there any studies that show the effectiveness of learning verb conjugations beyond the principal parts? If yes, is this effectiveness only relevant to certain languages and not to others?