Legendary Pokémon

I haven't understood sth....
D/P enables you to trade pokemon from pkmn GBA games to D/P! Isn't that right? Can I send to LG for example a pokemon from 4th generation?

D/P only allow you transfer Pokemon from Generation III games to Generation V and not vice versa therefore you can't even send a third gen Pokemon back to R/S/E/FR/LG...

That's really bad... Unless I clone my pokémon, I won't be able to have they back...

Why would you want your pokemon in GBA verions when you can have them in DS?

If I would play with someone that has no DS, but a GBA... or if I would participate in another tournament of GBAs...