The square terminal isn't a replacement for your current setup, just another option when designing your business.

The square terminal uses the interface Square POS uses on a phone so it lacks the Grid layout for fast choosing items, so in my opinion, it isn't for a store with 100s or thousands of items as it will now have to be searched for or scrolled thru the item list.  It is a much more portable device so you could use it at farm markets, tableside at a restaurant etc.  everything is built in, the swiper, chip an tap reader, printer, touchscreen, so it is much easier to set up.

If what you have is working there is no need to buy it.  I have 3 Square Stands and a couple ethernet printers and I don't have a place for Square Terminal in my retail store, but I may buy it for my Farm Markets where I currently travel daily with multiple iPad minis and multiple Square contactless readers because in that situation it might be easier to have one device for portability.