They never intended you to buy both versions, you're supposed to get one, and trade with people to get the missing Pokemon. Though as the games decreased in popularity that became kinda hard. However, Diamond and Pearl do use the wi-fi for trading and battling, and it also supports voice chat. So any Pokemon you can't get in the version you buy, you can just look for someone online who does have the one you want.
Also, it's likely the older Pokemon won't be available in these games, they usually throw in some of the more popular ones, Pikachu, Vulpix, etc, but other than those few you'll need to trade with the older versions, or find some one on wi-fi with them. All the old GBA games are compatable, so if you have a fully complete Pokedex in Emerald, you can trade them all over to Diamond and Pearl. I also imagine that Battle Revolution will include some more of the missing Pokes.
its the same thing. if you bought them all then you wasted 200 dollars. pwnt. the worst thing is that they come out with 2-3 differen't versions of the same game. year after year after year. *sigh* but if you haven't bought one before it is a good game.
That same thing can be said for almost any game sequel. Pretty much the same thing every time with small improvements here and there. The series definatly needs some bigger changes in some places. The graphics, mainly, but other than that, why change something that works so well?
And hey, is it just me or did Pikachu actually say "Pika" in the Battle Revolution video? Could that mean they're finally using the proper voices, instead of the 8 bit sound samples?