And in what particular way is mesh slow? In my eperience they load more or less immediately compared to sculpties which take their time as they are loaded in as textures.
No one said mesh was rock stable. As with all things it takes time for things to settle in and bugs be ironed out.In the end we will have a better experience.
About the advantages of meshes you just do not seem to know how they work. To keep this short, the basic advantage is that creators can make more advanced things with less polygon geometry compared to sculpties and regular prims. Meshes also make texturing easier and more efficient if the creators knows what they are doing.
If there is one issue with meshes it is the last part, people must know what they are doing! Lot of creators in SL started with making sculpties and use the same techniques to make meshes. They need to relearn their workflow to make better meshes that don't use so much geometry detail where not needed.
There should be plenty of threads in the forums for you to read about mesh/sculpt/prim comparisions, so no need to repeat everything here.
And about the update issue I would say you are making an issue out of nothing. Would you prefer the old days when whole of SL would be offline most of the day once or twice per month for updates? Much better now as sims go offline for 10-15 minutes unless there is problems. All visitors have to do is tp somewhere else and come back later.