I swap my E46N Givi top box from my Concours to my Wee so it's ALWAYS with me, and has been for 37,000 miles on the Concours, 2,000 miles on the Wee The Concours side bags are also plastic with aluminum supports, and not any more heavy-duty (probably less) than the Givi's, and few Connie riders ever take them off. I never do. Usually have about ten pounds or so of crap in them, also. Every now and then some Connie rider will report a lock wearing out and a bag coming off, but it's rare. Mine are bolted on for that reason and also to deter theft. So far, no noticeable wear on anything. YMMV.
2007 ABS Wee-Strom "The Silver Surfer"
2003 Kawasaki Concours "Mabel, the Flying Kaw"