I could not disagree more. If you want to play mindless iterations of tower defense or many other mini games, then sure, go ahead and play on your iOS or Android device. But if you want to play actual good games with a real storyline, then spend then $20-$35 on DS games. Good games will cost more than $0.99-$5.
I think this is true of some DS games, but not all. It's possible to buy a very crappy DS game for $10-$25, and it is possible to buy a really great game for under $5 on iOS / Android. I think iOS / Android is the way to go especially if you are looking for Internet Multiplayer games. I did get quite a bit of enjoyment out of playing a few DS games, but it is yet another device to carry around. In my experience I play the DS a bit, then it never gets picked up again for many months. The few times I wish I had my DS to play, I either don't have it or the batteries are dead and I forgot to charge it. I do not have similar issues with iOS / Android games.
I agree with you that there are many crappy titles for the DS, but all consoles have to deal with that. Even if there were any games that I would actually want to play on iOS or Android, the battery prevents it IMO. I think battery tech isn't where it needs to provide hours of gaming entertainment, especially online gaming.