I think that's becoming less of a norm. More and more companies seem to be moving to 4-year vesting, distributed annually.
What does it cost to transfer stock in the US in a privately held company? Here in Europe it can be up to ~$1K to transfer stock in a GMBH or BV, other countries more or less depending on the local situation regarding notary public roles and rates. In a public company it is dirt cheap but this is prior to IPO.
That + accelerated vesting in case of wrongful termination or termination without cause can be reason enough to batch share vesting events.