Jazz I have got 2 krytonite locks and I never used then,now only used to lock the bikes in the garage.I carry 2 £1 locks basically for popping into shops.I think you have got to look at your enviroment and street knowledge and that will tell you if you need to carry a lock.To me they are just excess weight,if I had to carry a lock then it would be a proper motorcycle lock,if I am going to carry weight then i will go the distance .Your rack will probably be ok if not overloaded,but I cant understand why the shop sold you a beam rack when the tricross takes a proper bolt on rack really well,I think I would be questioning the shop.If you do take the rack back and they give you your cash back then another option is the carradice bags, these are getting really popular again and there would not be a lot of cash difference.I can understand why you went for this rack having the choice to easily take it off,you can also do this with carradice, check out utube,sjs cycles and spa cycles.
hope this info helps
jackthelad