I have been putting them aside and buying more every week from Xur. I have enough coins to get another 19 if I want to spend them all at once. Since I have around 40 Exotic Weapons and Armor I will need a bunch when HOW arrives. I much prefer exchanging motes for exotic engrams and dismantling what the Cryptarch gives me from them though. It will be a huge start to use them right away and upgrade everything. I have even done some of the easy bounties like the one for the SGA just to get the exotic and shard them too.