Tubes are generally more prone to microphonics when they are exposed. For this reason alone, they should be inside the chassis.
Raquel your comment makes the mistake of assuming that audiophiles care primarily about the sound quality. Remember this is the same demographic that has a tendency to shun DBX testing - as the saying goes "seeing is believing" rather than "hearing is believing".
A fairer statement is that the sound has a lot to do with this hobby but it is far from being only about the 'technical' nature of the sound, like Don says
It's the fetish, not the sound. Same in the car world.
and there is absolutely no doubt about it...aestehtic consideratoins play a major roll and for some audiophiles it clearly dominates their perception of good or bad sound.
Tubes do look really cool and retro like - they are always guarenteed to elicit comments from anyone if they are visible, which leads to an enjoyable conversation about audio gear & music, often leading to greater anticipation and listening enjoyment. Also tubes can sound extremely good and no doubt sound "perceptively better" when on display, as a little microphonics never hurt anyone once they can see the glow.
"Perceptively better" is the same effect you get at a fine restaurant where a well dressed reverant waiter, the aesthetic food presentation and ritualistic behaviours can enhance the enjoyment of the food...and actually make it taste better...yes really!
Clearly the mind is strongly influenced by preconceived notions, visual and other cues, which is understandably why the rigor of DBX testing spoils things for many people - it takes away the magic.