Low-cost phototransistor to MCU interface
If you want to avoid the output of your phototransistor being anywhere between GND and Vcc, the solution is simple: add more gain. Just follow it with another amplifier of any kind. It doesn't need to be a Schmitt trigger, though the hysteresis it adds helps avoid rapidly switching between on/off states when the phototransistor is near the threshold.

Alternately, consider using an analog input, if you have one. This would not only allow you to detect light/no light, but how much light. You could then adjust the sensitivity of the phototransistor easily in software.