The question why does have to be asked,all you will be doing is changing the width of the BB,The shells will still be the same width apart as the difference will have to be made up with a 2.5mm spacer each side.

Perhaps in the future you want the ability to use 68mm specific external bearing setups too, like the THM-Carbones DP 68.

All sorts of reasons...

The only thing I'd consider is the depth of the thread, you'll obviously be losing 5mm of it!

Ask Tim Ti29er LBS to file it down while they are doing his brakes

Nick Evans wrote (see)

Doubt they have the tool, they'd try using their finger nails