Used to ride a lot w/small backpack w/2-5 kg weight; I also see some commuters toting med-size backpacks. If the back & butt holds up there's nothing wrong w/it IMO. But you're more of a hard-core rider after all. Many older or weaker cyclists might find that toting a heavy backpack causes back strain &/or butt pain. & often folks tour on saddles that don't fit perfectly even when using racks...adding more weight to body won't help there. Sure a backpack spreads weight vs using rear-only pannier but I've never had a rack or bolt fail. But I'm fairly light, never had a spoke break even.