Hi Krish 
long time, hope to meetup in person again sometime soon!
So we originally thought the number of active subscribers was the best value metric for us, but a couple of years ago we switched to MRR-based pricing, where we charge 0.25% of MRR (unless you are on a custom enterprise plan). The main reason for that change was that we were getting a lot of B2C companies using our product with very high subscriber volumes and low revenues. So our pricing model sort of broke and we ended up discounting too many deals. On the flipside some B2B customers had huge revenues but very few customers and were on our lowest pricing tier of $35 (or whatever it was at the time).
So shifting to MRR-based pricing allowed us to eliminate discounting from our operations which has made managing our business just so much more pleasant. It also better aligns our pricing with our mission which is to help subscription businesses grow their revenue, so if we’re successful in helping them do that we also get to benefit from that upside (used right, ChartMogul should help inform decisions that result in inflections well in excess of 0.25% of incremental MRR).
The introduction of a free plan was in direct response to our integration partners like Stripe, Recurly and Chargebee (of course ) introducing (sometimes lightweight) subscription analytics into their billing offerings. When a company has under $10K in MRR then every penny counts and paying for advanced analytics probably doesn’t make a tonne of sense over using something that’s free and workable with low volumes of data. So our free plan (which is free until a company has over $10K in MRR) is to encourage startups to start using ChartMogul right from day one before they even have their first customer, for us this is a really exciting time and we never want to stop working with startups at their earliest days.