My basic newb thinking was that iirc, digitalwrite at high was = to 5v, so with some quick proof of concept testing i thought it should do the same thing.
The difference wasn't explicitly stated: the I/O pin limited the current to the transistor, the 5V supply did not. Neither is a good thing, because the I/O pins are not designed to be current limiters. As already stated, you should always have a current limiting resistor connected to the base of the transistor (in cases where you are using it as a switch.)
With a current limiting resistor in series with the base, the I/O pin and 5V supply will work the same for turning on the transistor.