Yes, you can use Kraken Direct to purchase bitcoin to pay for goods/services. However it is very important that you do not put the merchant's bitcoin address into the "To" field. Merchants provide a price quote for a very short period of time (15-30 minutes) and if the coins are not received within that period time the order is canceled.
Because purchases from Kraken Direct take four business days to be completed your purchased bitcoins will be sent to the merchant but your order will have already expired. When this happens it can be very difficult to get your bitcoin back. You will have to contact the vendor directly and get them to return your coins. Once they are sent by Kraken Direct there is nothing we can do to recover your coins.