Practical benefits of expensive jockey wheels? Today I had to make a detour to the bike shop as a jockey wheel decided to fly out of a friend's derailleur. The shop were able to supply a replacement retaining bolt free of charge, however the only jockey wheels that they stocked were cartridge bearing ones at £20 a go. Needing to get home, the derailleur received an unexpected upgrade.

I did not notice any speed improvement on the way back, the jockey wheels didn't strike me as being featherlight and the shifting worked the same as before. I was beginning to wonder 'if we had been had'.

So, what is the point of expensive jockey wheels? I can see the benefit for the shop, but not for the cyclist.
The advantage of sealed idler cogs is that the bearings stay cleaner and don't (in theory) need lubing.  This keeps the cogs and derailer a bit cleaner, allows you to reduce maintenance slightly, and presumably provides a 0.0001% reduction in drive-train friction.

(I suspect that there was more of an advantage prior to indexed shifters, where considerable sideways force from the cog was needed to help shift the chain.  In that situation wear on the bearings lead to poorer shifting.  With indexed shifters (and the associated profiled sprockets and "climbing" chains) this advantage is somewhat negated.)