Level shifter vs pull up resistor
You may not be be able to pull up the output of a 3.3V chip to 5V. But assuming that your desired direction is possible, the other issue is that the rise time resulting from the pullup resistor charging the capacitance of inputs and stray capactiance is potentially very slow compared to an active pullup, and there is a trade-off between current consumption in the 'low' state and rise time. 

With a level shifter you can have very low static power consumption and high speed even with relatively large capacitive loads. For example, a 10K pullup and a 50pF load represents a time constant of about 500ns vs. the typical 4ns of a 74LVC245 (100x slower), again with a relatively large 50pF load. In some applications, that's not a big deal, in others it is.