You use strange coins for heavy because you have more strange coins than you need. I keep up to 500 heavy synths at any time - 100 on each character, 200 in the vault. Why? Why not? Rainy day stockpile. I top off each week from Xur or Varicks depending whether I am lower on coins or glimmer. Most exotic engrams I get via 3oc drop at 310. I got a 310 Stag (edit: not Ram) yesterday I used to infuse my Saint 14 up to 305. Xur is not as exciting as he used to be, but is still a valuable vendor. Certainly not the most useless in the tower or reef.