For example, Centurylink lets you pay online via debit card with just the card # alone. No CVV, address, expiration date, etc. For low fraud industries, this is a way to leverage very cheap debit card rates (eg $0.65 flat for small bank/credit union debit & 0.05% + $0.21 for large banks that are regulated debit), while making it simple to use debit cards compared to a credit card. If Stripe were to offer something like this, it would be a harder sale to make when I come in and review their phone/payment processing/ISP bills and pitch them $XXX savings.
Another thing is on credit, not offering over interchange pricing essentially limits you to only working with very small businesses. A supermarket I work with (4 employees including the owner) does $100k volume a month average, and is paying only 0.83% to 0.85% ($830 to $850 a month in total fees on 100k volume) a month, how can Stripe get anywhere close to that?
These types of issues could limit Stripe to a small & shrinking niche long term, especially with Mercury offering similar APIs with more features (though less accessible) at rock bottom pricing, and Heartland, Gravity and others piling on in a race to the bottom. IMO the reasonably easy to access & inexpensive API is the differentiator keeping Mercury viable.