I agree that the SA in your situation was rude but this was different. You're right about professionalism also! I think she was admiring the bag and was curious.I on the contrary think it is quite rude. Was in Melbourne recently and in Versace an italian SA heard me speaking italian and asked me and quote "Why didn't i buy a Versace bag that was on sale when LV are disgusting and crap quality" I was kind of shocked. A) You sell Versace - You should be selling me Versace.... NOT craping on another brand.... and 2) You shouldn't even ask about what other brands you own.... Its highly rude. If someone was in a restaurant and asked you how much you payed for your bag while serving you your bread. You would find it offensive.
My SA's at Gucci and LV know i buy other brands and KNOW how much they cost. These SA's ask because they want to tell you of for not buying their product and also they want to sell you their 3 grand bag that is probably no where near your style.
BUT long story short. I do think its offensive to ask about a mother brand and ask in a rude tone. Professionalism in the Work Force has slipped so much in the past 5 years. Its quite sad