The thing you have to do...if a cake mix passes the use by date, add a little baking powder...the biggest reason the cake mix is dated for "x" date is b/c the lifting agent (usually baking soda or baking powder) eventually will no longer lift the cake, and only getting egg lift only gets you so far. Now, this doesn't mean you can use 10 year old mixes (b/c other products will start to go rancid in the mix - nuts, oils, etc), but they should last 6-18 months past the use by date (depending on flavor - nut mixes will be short for the aforementioned rancid oils reason) with no decrease in quality if they get this additional lifting agent added.
And if your cake mix is the dry goods - flour, sugar, lifting agent, cocoa powder, and you add your own oil, eggs, and nuts - what in there is going to go bad? I don't know about anyone else, but I bake with four year old baking powder all the time, and I've used old cake flour and sugar too - eventually you need to toss baking powder - but it takes years.
I do throw out things I've had for years, but I don't throw out a cake mix that is six months past its "best by" date.