As a someone looking make purchases with BTC; if the value of a BTC is going up, why would I use it to buy anything? It's gone up $700 in the last week; up $200 in the last 24 hours alone. I'd feel incredible silly spending bitcoin in a growth phase.
Why do you spend any currency when you can buy Bitcoins instead, let it grow and then sell them back into the original currency?
Any reason you can give applies to spending Bitcoin.