You know, life gets pretty hollow when everything is 'billable time'. It's also fun to just hack and tinker.

Also, if maximizing billable time is the goal, maybe the guy learnt a lot playing with another UNIX and increases his worth :p.

You've put it better than I ever could. It's frustrating to see someone share an experience along the lines of "I earned X this weekend by exploiting a niche I completely ignored before and learned loads!", only for a commenter to inevitably come along and say "well, thats all well and good, but if you value your time at Y per hour you've actually lost money, not to mention the wasted opportunity cost of not using that time to become a rocket scientist and earning even more!"

Well, you can't have it both ways. If you (in the general sense, not you specifically) do a project for your own reasons, great. The problem is when you try to make an ROI case that only represents one side of the equation. If you don't want people to poke holes in your ROI argument, then avoid framing your projects in a way that creates opportunities for that discussion.

₹35000 (rupees) and not $ 3500 (US dollars)!

It saved him from buying a new laptop (he could have also only upgraded the ram)

The motherboard of that machine doesn't support more than 3 GB RAM.