Yes. Normally a user filling in a form is the last step before conversion (signup, purchase, etc) so any users you lose at that step are horrendously expensive,

Yes. Normally a user filling in a form is the last step before conversion (signup, purchase, etc) so any users you lose at that step are horrendously expensive, good client side validation can make a significant difference. Validation for the purpose of having clean data in your database is obviously important too, but less so. But obviously if you had a service for providing the first it wouldn't be hard to offer it as a server side services as well.