DS is simply the platform any RPG fan out there can't ignore. Especially if you're a nostalgic old scool RPG fan.
Dragon Quest 5 and 6 never got a release outside japan, and the fact that draque IV already has pieces of european languages translation hidden in its japanese version hints that it's surely gonna hit western shores (it already did with a NES release in North America in 1992 under the Dragon Warrior 4 name)
So you could well imagine that 5 and 6 will follow as well.
It's really interesting for them to remake them for the DS, because some of us never got to play them. I'm talking about the european RPG fans specifically who didn't even get the fourth installment for the NES.
The eighth episode for the PS2 sold very well in Europe and was critically and commercially acclaimed, so it would be silly not to release every new installment of the main series in western countries. I'm really happy about their decision to remake them. I'm playing the fourth in English thanks to action replay which unlocks the other languages, and it's really good.