When you've been a programmer for quite a while, it's something you begin to understand. It's a gamble, you're working for free, and most of the time the "founder" substantially overestimates their worth in the company and reflects their ownership percentage in such a way.
I'd say about 80% of the requests I get are "you'll get paid if it works - equity style" and as most startups fail, it's not a lucrative strategy.
+100
99% of equity or rev share agreements don't make sense for developers. Founders who know what they are doing shall have no issues raising funding (or fund using their own and family money) to pay for basic MVP development.
On other note - just noticed your signature - are you based in Watford? Reading about your project now..