I bought an iPod touch cheaper than the 3DS or the Vita for playing games on, you don't -have- to buy an iphone to play iOS games. Only real potential problem is that iPhones are always more powerful than touches, but, realistically I haven't really ran into many games that I couldn't play on mine.
They also are rendered irrelevant a hell of a lot faster: I got an iPod Touch 4G partially because of games, but then a year later GTA3 hits and it constantly crashes. I have a Nexus 7 but Android has a problem of being overlooked for games at times, though it looks like that's improving (probably BECAUSE the Nexus 7 is the potent, cheap alternative to the iPad), but unless progress very rapidly stalls out I can see tablets/smart phones being way more costly albeit in a different way than consoles to keep up with gaming. Especially the iPad, those are usually $500 damn dollars and to stay relevant for games you'd probably need to upgrade at least everyone other year unless you're just playing Angry Bird caliber games, which this definitely is not. I suppose you could just keep getting games cheap or for free, but the free ones you have to question the quality of (or the pervasiveness of IAP), and either way that'd probably still come out way ahead of just buying a 3DS depending on what you're buying (and that may still be a better GAMING value depending on tastes even after buying $300 of games or whatever.)
I'm generally for playing to a platform's strengths and as such staying exclusive, but we're talking more traditional style games we got on platforms we didn't necessarily need to keep replacing (well, PC certainly fit that bill, but progress slowed there), so I do hope they either get more port happy or hardware progress on the mobile front practically flatlines so a $500 iPad really COULD get me through many years of gaming. Then again that battery would probably start crapping out, so maybe it's more that Apple's hardware model is actually just plain abhorrent for a gaming platform.
EDIT: Dammit I meant 4G, not 3G.