Serving s21 to new tenant

Possession - Section 8 and Section 21

Hello,
Is it a good idea to serve a s21 notice to a new tenant? When things are friendly and they will sign it as a receipt.
Does the notice last forever or until a new tenancy agreement is made?
Is there a reason why this is not a recommended practice?

Of course I would explain that it's just a procedure and my intention is for the AST to roll on as a periodic tenancy.

David

PS I ask this now as I have just had my first eviction using a s21 notice, it took nearly 4 months and was unpleasant.