Should You Build a Feature Just to Win a Customer?
Startup Codex

This is a question that all B2B startup founders have to answer at some point.

How much deviation in the roadmap can you accept to win a new customer?

This very pragmatic post proposes some good arguments on the conditions under which this kind of deviation can be quite acceptable, and those that justify refusing it.

Does the wise advice to always say no to sponsored features would also apply to a SaaS startup selling to mid-market (ACV 20k to 50k+) and has yet only 2 clients?

No. This is not wise advice, it is bad advice in many cases. As long as the deal size is big enough.