Well, if you look at professionally made amplifiers, they all have covers. They must have. Otherwise, they are unlikely to be certified as safe to use. Sometimes, you can dismantle covers to experience the warmth of the sound, but that is on your own risk.
For showing off and impressing your friends with an t00b amplifier, I can kind of tolerate, but for a general purpose lab instrument - hide the tube inside, make the instrument inherently idiotensicher und bombenfest.