Practical benefits of expensive jockey wheels?
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.)