Fail fast.

I can perfectly well install, set up and maintain my own Ruby servers - but it takes time.

Alternatively, I can pay someone else to do that, and remove that timesink from the elapsed time between "start developing" and "find out how well we've achieved market fit".

I can always optimise later - move off Heroku, develop our own load balancing, all that stuff. Once I've got a working product/market fit, I probably will.

But doing that before I know if I'm going to chuck the entire infrastructure in the garbage and move on to idea #2 (and #3, and #4...), or indeed pivot so wildly that we'll have to reorganise all our server stuff anyway, is a waste of time. And time is valuable.