I had some of the first beta Proofgrade that had a core of wood. It worked for some things and others it didn’t. Often when we think of high quality plywood, we think of the layers as being all wood because it gives the most strength across a wide variety of use cases.
When it comes to making a consistent product that works in a small CO2 laser for a variety of use cases, the values that go into calling it quality are different from cabinet grade plywood.
For example. Trying to get a consistent base surface engraved into an all wood plywood is very difficult. With an MDF core you can be assured that the inlay will go in flat.
I had a conversation with someone the other day about plywood being an inferior “cheap” thing to make cabinets our of. The best he said were the old cabinets that were of solid wood. I told him that I would prefer using high grade plywood because of the stability that it brings. But definitely no chipboard stuff with vinyl facing. That really isn’t plywood.
Another issue is needing the plywood to remain dead flat in the bed for precision cutting and engraving with defaults. Often veneer cored three ply plywood can be unpredictable in how it behaves in low and high humidity environments.
Getting an inner core veneer that is consistent, integral, no voids, knots or other surprises might have kicked up the price, I don’t know.
These aren’t all the issues but recognizing that there are other values and end cases to consider that might be better served with an MDF core and the decision isn’t made because it is cheaper. Although that might be a consideration.
My experience with Proofgrade is that it is a quality product that works great in my Glowforge. I haven’t found anything that so predictably works like this. Especially in that it is finished.