Car runs cold. Is it bad?
 

So, I've had my car for almost two years now. 2004 wrx.

Ever since I've had it, the temperature has been about 15 degrees C below the middle area (sits 1/4-1/3 above the lower middle mark in between the hotter middle mark)

Through out the life of the car, I've replaced the following that could be related (not to fix, but for other reasons)

STI Cluster (gauge has hit the high hotter mark one time when radiator went)
I recently switched out:
timing belt
full coolant flush (OEM long life w/ conditioner)
water pump (OEM)
thermostat (OEM)
radiator hoses

The only thing I can think of is that my heater hasn't ever worked. Eventually I'd like to switch out the heater core myself, but am a bit intimidated and have other things to attend to.

So is this bad for my car? Still runs really well with 190k+ miles. Never fluctuates in temperature, but I feel it's better for it to run a little warmer.

Thanks for checking it out! All input is greatly appreciated.

Anywhere between the two marks is its proper operating temperature.

Cool, that's what I thought, but just wanted some opinions. Thanks.