Up until the last few decades Harley-Davidson used a paint on the inside of their aluminum engine cases because of the porus castings to help keep oil "weeping" through the case. I had a boss back in the 80's who also claimed that a rough casting was better for releasing oil from the walls of the engine. The little casting "tits" helped form droplets when the engine vibrated (no Harley vibrating jokes needed). I've used glyptal before, but for the most part I only see it needed as a sealer for a porous castings as mentioned above, good looks that you will cover up and to help seal any small particles onto the walls so they don't ever come loose. If you have properly cleaned your engine before and during assembly, I see no need for it. You must properly clean your engine before using it anyway, so I think, for the most part, it is a waste of money and time. No doubt it looks good though. The benefit, in my opinion however, is very small if at all.
