I can't disagree, but I'm happy to be the guinea pig for a solution that is readily available...
As one who previously tried a readily available alternative (which wasn't terrible, just a little too runny), I learned my lesson and am now happy to pay a few dollars extra to let Maine Sail be the guinea pig with close to 50 other manufacturers.
The physical properties of butyl tape, like any other polymeric product, is a complex blend of ingredients (like MS already mentioned) and other attributes such as molecular weight and amount of branching in the molecule. As with any sealant, the key tradeoff is always cohesive strength vs. adhesive strength. By tradeoff I mean that you ALWAYS sacrifice one when you improve the other, so the specific application will determine the proper balance of properties.
You don't need much adhesion for objects that are bolted to the deck - you just want it to stretch like crazy and stick enough to maintain its seal without letting go. Any more stick is more than you need, and will result in you sacrificing some stretch and cohesion.
MS has tested a whole bunch of products to get the one that has just enough adhesion to hold, but will stretch like crazy (check his videos for an eye-opening demo), so when you pull on the lifeline and the stanchion is stressed, its baseplate will never lose its seal because the bedding will stretch without losing adhesion.
This balance of properties is different for marine applications than it is for anything else, so by buying his product you have the one that's best for its intended purpose.
...and is quite cheap...
"Cheap" and "marine" are rarely compatible, and in this case the cost of a roll is far less than the time and effort you put into using it. Butyl tape that is truly marine grade is one of the cheapest things you'll buy for your boat, and saving a few dollars with an inferior product is false economy.