The drawback is that every time the entity is used in a new transaction the database is queried.
Lock and IPT are two separate things.
You can have IPT with a pessimstic lock or an optimistic lock.
IPT only means that there are multiple instances of the entity bean, for that particular primary key, in existence.
Optimistic lock means that you assume no other transaction has modified the data between the time your transaction loaded it and commited it. This is achieved by using a version number, timestamp etc.
Whereas with a pessimistic lock the row cannot be updated by any other transaction till your transaction completes.