Colsie, there are other factors of interest. Speedbrake causes vibration throughout the aeroplane. With flaps extended it gets worse. It can be quite sobering to sit behind the wing and see what speedbrake does to the flaps. That is the reason the use of speedbrake is not recommended beyond a certain level of flap. Flap 5 on the 737 and full speedbrake is not a nice combination!
Another reason is that you are throwing away efficiency, and I view it as an admission that you have misjudged the descent if you need to use speedbrake except where you are having to obey ATC instructions. If you have free reign over your descent and use speedbrake, you are throwing away energy, hence you have wasted fuel. It comes with practice!
Another factor may interest you- using partial speedbrake on the 737 is not recommended because it affects the roll control and makes the aileron control more sensitive. However I don't like using full speedbrake, especially when any flap is out, so I only use what I need and take care with very gentle controls.
Right- are you ready for your exam?
(May I say how nice to see such a well phrased query? You are very welcome to ask more! Do you have a further interest in aviation?)