Considering the fact the modern humans were second iteration of that kind of life form (re-created by ascended Ancients after the plague) and they "evolved" (???) on Earth, it's safe to assume that Earth was extremely important to the Ancients. The backup gate sort of gives that away, as there is no other planet with two gates.
And it's quite probable that Ra DIDN'T bring the gate to Earth (just bragged so). Remember, SG-1 builds on original Stargate, but explains a lot of info from movie as Goa'uld propaganda.
But I think assigning some extra value to a symbol is going a bit too far. It is an unique PoO for Earth only (duh), but nowhere it says it can't be a coordinate in an address.
Decoration in Atlantis may actually be meaningful, but in a different way: Atlantis was built on Earth and it was considered a crowning achievement then, thus marking it with a symbol of homeworld makes sense... In my opinion it would be similar to displaying a crest in an old castle - it can be put on everything, including silverware.