Also, its cheaper.
Just think about it, here are the prices from wal-mart
Resin-$14.95 per 1.5L can
Bondo-$9.99 per 1L can
Cardstock-$11.95 for 250 sheets
Hot glue sticks-$5.95 per 100 sticks
Therefor, say you need 1000 sticks for your costume, 200 sheets of cardstock, 1 can of bondo, and 2-3 cans of resin. The total price would be around $110.00-$120.00.
If you use the resin/fiberglass:
Resin-$14.95 per 1.5L can
Fiberglass sheet-$9.95 per 1M squared
Bondo-$9.99 per 1L can
Cardstock-$11.95 per 250 sheets
Now again, lets say you need 200 sheets of cardstock, 2-3 cans of bondo, 4-6 cans of resin and 5-6 sheets of fiberglass sheet. The total price would be around $170.00-$190.00.