Is spending $0 on marketing good or a missed opportunity?

I've seen a lot of excitement on Indie Hackers about growth without spending on marketing. A typical statement is: "We have spent $0 in marketing and made 500 sales in 7 months". This is usually done via social media channels such as Twitter.

First, that success is amazing and truly validates the idea. However, there seems to be an ethos that spending on marketing is bad and should be avoided - i.e. the common statement of $0 spent. I don't believe this to be true...mostly...and see it as a missed opportunity for growth.

In the ideal situation, every $1 spent on marketing has a positive ROI, such as $2 revenue. If you can find that growth engine you should pump every penny you have into it. So in the example above, those 500 sales "could" have been 5,000 sales if some of the revenue was put into marketing.

Another situation is for every $1 spent the ROI is $0.50. This often occurs when you start marketing since a lot of keyword and ad experimentation is done. However, this is not sustainable and eventually need to have a positive ROI.

Of course if you don't have revenue and your users are free then there can't be positive ROI on marketing, unless the goal is purely having more users.

Overall, marketing can be a great thing* and ignoring it could be a big missed opportunity.

*but expensive it not done right.