Why outlet makers warn against aluminum wires?

Daddy0
I believe the plastic cone wire nuts you reffering to are the MARRETTES as they called in the trade.
George

Aluminum wire will vibrate with the frequency of the signwave. In this case, 60hz. This causes it to loosen from any connections other than solid crips. When it loosens, you get arcing at the connection. It's also is soft so it will loosen up over time. You need to retighten at least every year.
Al wire is not compatible with copper and cannot be connected together without special al/cu connectors. They do make a grease for this also.
Al wire must be sized larger than copper for current carrying capacity (usually one size bigger eg; #10 copper you would use #8 al.)
A lot of trailers use al and older homes. They used a special al/cu outlet for this purpose. Inserting copper blades from the plug has no effect. You can plug most any connector wire into an outlet with no problem. Just because the conductors are al doesn't mean the plug connector is. It is usually copper(or an alloy.)

When multiple dedicated circuits are used for audio/home theater is it necessary to run all of the 110V circuits off the same side of the 220V feed?

Here is a little reading material regarding AL terminations and AL to copper terminations.
Lit is a little old, but still to the point.

http://www.inspect-ny.com/aluminum/piclib02.htm

The building where i live is a prewar one.