You never want to be doubled. Ask Hector.
I have a Hector. And a Lukas, a -SPD Nowi, and so on. It's a thing I've accepted working around with my infantry choices. I'm not saying go for Hector speed levels but just a few points since the top end units will still double him anyway.
Plus, I hate planning around +SPD units to figure out SPD as a defensive stat.
I mean yeah, in arena I feel like you're either a 40+ speed monster or don't bother. But +SPD Ike's in that range where with hone speed he can at least avoid being doubled and then counterattack with quick riposte. And he'd be able to at least double a few characters in order to get some more mileage out of Heavy Blade. Which is pretty kickass with Luna based on what I've been doing with Ike so far.
Yeah, that's true. The only notable unit I can think of I'm actually worried for Ike getting doubled is probably Nino? To be fair, I haven't simmed out what Ike looks like at 40 for matchups so I don't know the specific breakpoints to care about.
Ultimately, my hope for Ike had him bulkier than I think he ended up being but I'll see how that pans out in practice when I get him to 40.