The balance shafts are there to equalize vibrations that are an inherent problem with 4 cylinder engines. Are they absolutely necessary? NO
Is it a good idea to install the belt? YES
While it is not crucial, it smoothens the feel of the car. Some cars such as the new Type Rs do not have balance shafts. They sacrifice a bit of vibration in exchange for less drag on the timing system.
Excessive vibrations may cause premature wear on other components, most notably the mounts.
Just get the $20 belt and slap it on.
Note: the rear balance shaft is not directly driven by the gear you see, so although your alignment mark may be correct, it is possible that the shaft may be off. I suggest you stick a hex key in the hole in the rear to hold it in place while installing the belt. If the engine idles or drives rough, remove the belt and spin the rear shaft 180 degrees and then reinstall the belt once again.
Hope this helps.