About 15 to 20 years back, bats were high in moisture content and the blades were thinner, so, a bat had a chance to dry out and lose moisture if left out in hot conditions, with no protection. The result would be a thinner bat with very less moisture, thereby making the wood very brittle and susceptible to break easily. Oiling was hence mandatory. Not just while knocking it in, but as regular maintenance, typically once every season.
Modern day bats are a whole lot different. They are dried more to make the cleft light so that the makers can have those big profiles on them. With pretty little moisture left in the willow, there is hardly any left in the bat to protect. That is not to say that there is no need to oil. It is all the more essential to ensure that whatever moisture is left, is protected. The wood may be brittle and there will be surface cracks, no doubt, but the bat itself may not break thanks to the thickness of the blade. This may be the reason why we get more of edge cracks these days compared to older bats. But despite the edge and toe cracks, the bats serve well as long as they are taken care of.
In a sense, it is essential to ensure a coat of oil on the bat, no matter whether is it is old or new. Having said that a modern day bat will get away without much damage if it is not oiled, thanks to those thicker profile blades, all other parameters kept similar. Finally, professional players have their kits looked after by a representative of a bat maker. They have multiple bats in their kits, which go through periodically maintenance or refresh. All of that is free for them, thanks to the logo that goes on the bat. So, they don't care much if it is oiled or not. For them, the balance, the comfort, the ping and purchase of their shots is what matters; unless someone is way to fussy.