I've played snare for a few years now and all my teachers recommended that I use double strokes for evenness of sound and speed. When I started playing some more difficult timpani parts however and used double strokes so I did not have to move my arms from drum to drum, thinking it would be a clearer and more even sound, the conductor told me to use single strokes instead. Why is this? What causes the difference in preferred sticking for snare and timpani?
The primary reason for this difference is in the mechanics of the drums themselves. Snare drums have both top and bottom heads that are usually tuned to a relatively high tension compared with the timpani. Hence, all the notes are extremely staccato and have virtually no sustain to them at all, so it is very difficult to play a note on the drum in the 50 or so milliseconds while the precious note is dying away. If you want to get exactly the same sound, you need to produce the next note under identical conditions to the first, which means hitting the drum in the exact same place with the exact same force.
Timpani have no bottom head and are designed to resonate, and it is easy to play notes that take almost 2 seconds to fade completely. This occurs because the parabolic nature of the timpani shell reflects the sound inward where it can continue to bounce around the inside of the drum. Since these vibrations last so long, if you play a second note in the exact same place on the drum the strokes will sometimes cancel each other out by producing the same vibrations going in opposite directions. When this happens, the net result of the second hit is not a new note, but stopping the vibration of the first and producing very little of its own. In order to replicate the conditions of the first note, you need to play the drum in a different place so it is starting in a spot with no directly opposition sound-waves.
Since double strokes tend to occur exactly in the same place, they work very well for producing even notes on snare where this type of accuracy is required, but not so well on timpani where different notes tend to have a canceling effect.
a lot of teachers might claim that for evenness of sound you should use alternating sticking. this is highly variable and does not always apply though.
i think you're partly right in your answer, but there's nothing wrong with playing repeatedly in the same spot on a timpani head. there might be a bit of sound-cancellation but that can be useful and not necessarily noticeable.
the short answer is simpler: there's no reason not to use double-strokes.
in fact, there are times when you must. (so don't worry if you come across a place where you think you have to; do it!) there are places where some players use singles and others use doubles (the end of beethoven 8, for ex.). it can simply be preference.
a more technical answer would be that a bigger/higher stroke will give you a fuller sound.
say you have two notes to play. with two sticks, both sticks can start, say, 10 inches above the head and you'll get two "full" strokes. with one stick playing both notes (double-stroke), the stick will have to start lower/closer to the head, presumably 1/2 as close, so the height of the strokes will be shorter and you will get a less full sound. (this assumes a tempo that's fast enough to ensure you cannot raise a single stick as high as two sticks; hopefully you get the idea.)
all of this, as usual depends on the ability of the player, tempo, dynamics and all the rest. but i think it's mostly a question of sound from the player's perspective, more than the physics of the drums, although you make a good point in your answer.