Hah! Just got turned onto this thread, and what a ride
There's a lot of comparison with tapered wheel bearings. Doesn't apply here. Tapered bearings are pre-loaded and the tapered needles are constantly being pressed against the race, which in turn presses the race against its shaft or bore. If either race spins on its shaft or in its bore, then the bearing fails. The preload is enough to prevent the races from spinning and to allow the needles to rotate through the grease. In fact the grease serves to increase the preload by being forced between the tapered needles and races. There's usually a 0.0005" to 0.00015" clearance in a tapered bearing for this specific purpose. The grease is integral to that preload function (that's why we back off the castle nut before inserting the cotter pin on wheel hubs).
A pilot bearing has no such preload. The inner race rides on the transmission input shaft without any interference, it's a slip fit. So the idea is for the bearing to turn easily whenever any axial load is applied to prevent friction. If no axial load is present, there's no friction to deal with. If there's axial load, then the ball bearings need to allow the inner race to turn easily so it's not generating friction on the input shaft. That means the ball bearings require as little resistance as possible to turn, otherwise the input shaft will spin in the race and heat will result.
Typical wheel grease is too thick for this to happen. This is why they use a very specific, light grease that's pre-applied by the bearing manufacturer.
Does that mean the bearing will instantly fail if it's greased with wheel grease? Hells no. Most likely some grease made its way onto the input shaft anyways and any slip in there will be handled just fine for years. It's not a big deal. In that case the pilot bearing is acting like a bronze pilot bushing right up until there's enough axial load to overcome the resistance of the wheel grease in the bearing. That bearing is not highly loaded most of the time. All it does is limit axial deflection of the input shaft, it doesn't actually "bear" anything most of the time.
As for why the bearings are only sealed on one or no side, it's the same reason; to limit turning resistance. A seal on one side typically doesn't add much resistance because the inner race can still shift to the opposite side to spin freely. A seal on two sides guarantees extra resistance because the seals ride on both thrust sides of the inner bearing race. They add a seal to the side that's most likely to be exposed to crap and dust.
I'd use a bearing with a seal on a manual transmission, to prevent clutch dust from contaminating the grease prematurely. A bearing with no seals would be fine with an automatic transmission. A bronze bushing works fine, too, on this application. On my manual-transmission Audis, bronze bushings have been a failure of epic proportions, requiring complete transmission rebuilds. Automatics don't even use a pilot bearing at all. But for some reason, on these Muncies and Super T-10s, bronze bushings work just fine and last just as long as the clutch (which is really the only length of time these bearings/bushings need to last anyways).