A lot of adhesives have some sort of goop dissolved in them. I used to work for a critical medical device manufacturer who made specialty equipment for cardiac surgery. We made our own adhesive for our own product, and we literally MADE it. There was a recipe, just like making cupcakes. We donned special safety gear, went in a special room, and mixed solvents and the materials we made our products from in the form of components that didn't pass inspection being chopped up into tiny pieces, and let it sit until everything was dissolved. In use, the solvent melted the plastics we were trying to adhere, welding the pieces together, and the dissolved material in the solvent would bond with the pieces being adhered as the solvent evaporated. It made a stronger, absolutely leak-free joint.

So- when you're gluing neoprene the edges aren't solid, because it's a foam. A great deal of your edge is air. If you have neoprene rubber dissolved in your contact cement you can theoretically create a better glue surface because the rubber fills all of the tiny bubbles in the edges to be adhered. In practice, however, I have found that it doesn't really give that much of a performance advantage, and it falls really short on cost, because the Seal Cement is more expensive and it absolutely will not keep. Once you open it you have to use the whole tube in within a couple of days or it's just going to solidify. I almost always blind stitch my seams anyway, so it's not that the extra strength is necessary. It's more the glued edge I'm after.

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