If you don't create any variations on the front-end using size, width, etc. calls, ProcessWire won't create anything except for the variations created by default for admin (preview) purposes. Third party modules may still create their own variations, but if you're strictly against that behaviour, you can always avoid using any modules that do this ?
What you're probably missing here is the amount of data: sure, browsers can handle resizing images, but if you're considering load times and amount of data users have to download to use your site, you absolutely want to resize images before they hit the visitors' browsers.
When you're asking if this behaviour should be disabled to "reduce additional http requests (if the user resizes viewport)", it kind of sounds like you've implemented srcset on your site – otherwise there shouldn't be a additional requests when the viewport is resized due to images. Even then if you start with a larger viewport and then resize it to smaller size, browser shouldn't load the smaller sizes. Only case generating additional requests is when you go the other way, from small to large – otherwise tiny images would likely look pretty awful scaled to a larger size.
Short answer is: no, it shouldn't be disabled. That being said, you don't have to create any additional variations if you don't want to – but be considerate of the user and don't make them download more data than they actually need ?