Not many Ike players (only a bit with River), but I do know the Marth MU from Ike's perspective because I used to main Ike for 2 years.
And from Ike's perspective it does not feel like an easy MU at all. I've played against Haze's Marth offline with Ike and whenever I gimped it felt like it shouldn't have happened. The best move for Ike in the MU felt like was Bair by far because its range to keep Marth out decently when I had space to do so and being his only aerial with fast start-up. I'm sure that DS has a hitbox at his head height all the time
http://www.youtube.com/watch?v=KzAWoZsGpwQ#t=164s .Retreating Bair doesn't feel super safe. Ike's main problem in the Marth MU (and other fast character MUs like Fox and MK, that's why I really hated such MUs) is pretty much what Blubo said. He outranges Marth but in order to be safe he needs a lot of room and Marth has a relatively easy time cornering Ike to make him unable to keep that safe zone and making almost everything he does easy to punish. Such MUs for Ike have always felt like "First try, hit shield but free from punish, second try, hit shield, get punished and wrecked". Ike's average to mediocre mobility makes it almost impossible to zone out mobile characters. Retreating aerials is important for Ike to make his moves safe, but he also loses stage control when he retreats and he is not very good at regaining it. And once he gets cornered it also becomes easier to send Ike offstage with Marth.
Watching
http://www.youtube.com/watch?v=IbYqY5fZ3zI it really also seems like Ike has nothing to intercept Marth's DS. 95% of the time Ike also doesn't really get anything from edge hogging Marth. Edge guarding Marth with Ike is like edge guarding MK with Marth (we can rack up some damage on MK offstage by mini edge guards or ledge trapping, but Marth can't ever gimp MK).