Re-balancing individual ETFs is currently a $20 trip at most brokerages, i.e. sell units in one to re-balance into the other. Done too often and those commissions add up. However, for someone who is accumulating, it is easiest to re-balance by simply adding new money to the ETF that is underweight and NOT selling off units of the one that has gotten out of balance. Secondly, there is no particular need to re-balance more than perhaps once a year or every few years, depending on the range tolerance one is prepared to accept, e.g. does it matter whether the VAB allocation ranges between 15-25% and not actually at 20%? Probably not because you don't actually know that the "right" allocations are in the first place. You are simply drawing your own lines in the sand.
Which brings up the point of the asset allocation ETFs. They will re-balance internally far more efficiently than an investor ever will with 4 separate ETFs. Humans are prone to indecision, hesitation and impulse. They always second guess. By giving that decision to an AA ETF provider, the investor can go on to more important things in their daily lives. Despite being a DIY investor for 20+ years, I would go 'all in' on the AA ETFs if I was starting over today. Indeed, I am moving (have moved) to AA ETFs in my registered accounts (one each in my RRIF and TFSA) because there were no cap gains consequences to overhaul those accounts.