I get what you are saying, but I feel reducto ad absurdum is unfair here. I understood the parent's point as this; using Sinatra to give yourself the convenience of the {get,post,...} methods for working with HTTP vs. Rails, which forces upon you its more opinionated structure for websites.
I agree with the parent, that I have no use for the complexity of Rails and Sinatra is convenient. Other people are free to choose as they wish, and Rails might be the more appropriate library is some cases regardless of preference (like in the sibling post about teams).
But there's a ton of utility on the spectrum from Rack to Rails and it's perfectly reasonable to choose a midpoint, and perfectly reasonable to choose it because you think Rails is too much but Rack is not enough.
I absolutely agree with you. What sticks in my craw is the oft-repeated mantra that "Rails is too heavy" or the perception that Sinatra is doing something that Rails isn't.
When you look at Rails as a collection of middlewares (which you can choose!) and support libraries (require 'rails/all' may not be needed!), instead of looking it as a monolithic black box that can't be configured to fit your needs, it's suddenly a lot less "heavy" and a lot more "wow, okay, buffet-style app composition sort of rocks". People seem to have this idea that Rails is on the far end of the "magical and heavy" spectrum, when it can be just about anywhere along the spectrum that you want it to be, based on your selection of middlewares and libraries to use.
I have production apps that range from small one-off Rack apps (my dynamic asset server, for example) to Sinatra to full-blown Rails, and they're all very similar beasts. Rails isn't some mystical black box, and Rack isn't just some RFC somewhere, and I think it does a grave disservice to both to marginalize them.
I agree with the parent, that I have no use for the complexity of Rails and Sinatra is convenient. Other people are free to choose as they wish, and Rails might be the more appropriate library is some cases regardless of preference (like in the sibling post about teams).
But there's a ton of utility on the spectrum from Rack to Rails and it's perfectly reasonable to choose a midpoint, and perfectly reasonable to choose it because you think Rails is too much but Rack is not enough.