BTW, when I was at the LBS the wrench told me that my chain was a bit too short because he couldn't get it to go big/big. I pointed out that I shouldn't ever be big/big!
Yes, but you want your bike to be able to do this, should you be in one of your less clear/ more tired moments and accidentally make that shift. If the chain is too short and you accidentally shift "big-big" while riding, it can literally rip the derailleur off the frame, potentially destroying your frame, at a minimum destroying the derailleur. A longer chain also just shifts better.