The issue is that a chain has a lot of strenght in tension, but due to the thickness the chain is not strong laterally (from the side). Cross chaining puts a lot of lateral tension (sideways flex) and will cause the chain to wear out prematurely. Also, being in the big-big combination causes the rear derailleur to be pulled tight, and imposes a lot of force on it. I've known people myself included that have ripped rear derailleurs off their bikes just while pedalling. i.e., climbing up that grueling hill in too high a gear and muscling it!

Plus, with all the gear combinations and possible ratio's, you should be able to find a similar feeling gear using the middle chainring and one of the middle cogs. Without the lateral flex imposed on the chain.

L8R