To say that Gusano de Oro is harsh is quite the understatement!
While there is a certain legitimacy (or at least history/mythos) to the inclusion of the worm, I'd say that for this product it's more a case of marketing. (Actually, having a background in Entomology/Zoology I've managed to identify numerous things in their bottles - including beetle larvae!

- not just the Agave Caterpillar which is supposed to be used!)
edit--
A quick side-story here... A few years back I read an online piece about using a Britta filter to make cheap vodka taste better. It seemed reasonable as the activated charcoal would removed impurities - hence some of the 'bad' tastes.
Well I had a large bottle of Gusano de Oro that had been given to me (notice the consistency of people giving this stuff away!

). And to top it off the bottle had been opened, one shot poured and then sat for a couple of years. So we're talking about as bad as it gets.
I recalled the Britta experiment and thought 'hmmm... I wonder if that would work on GdO?' The mad scientist in me thrilled with anticipation and off I went to experiment. After running the GdO through the filter 3 times there was little change. After two more passes the colour went from yellow-ish to grey-ish. And the flavour did indeed improve! Although when asked how good it was my standard response was 'well at the start it was disgustingly horrible, and at the end it was just horrible!'