Actually Chrom has this really weird level of importance. One thing is that Awakening was designed as if it was going to be the last FE ever and Chrom has a lot in common with a lot of lords, as if he was designed to be the epitome of all lords. Though the thing that really makes Chrom important is that he is a Sigurd expy. Sigurd is an important lord, FE4 defined what most of modern Fire Emblem is, but he can't really be used because he is a mounted unit. His son wouldn't work either because he is a myrmidon and the entire myrmidon move set is already being used for Marth. Chrom can be considered an out of the box method of representing FE4, and he also makes a good series rep simply because of the nature of Awakening's development. The biggest issue with Chrom's move set I can think is his recovery. I can't think of a move he could use to do so other than dolphin slash because he never jumps high into the air.
....what?
Okay, seriously. What FE games have you actually played?
And how the hell would Chrom represent FE4? That'd be like me saying that Roy is important because he could also represent FE5.
I would really like to hear your argument as to why Roy is important or unique and makes a good rep.
Not counting his appearance in Melee, he's no more or less important than any Lord not named Marth or Ike.
As for uniqueness, assuming you mean what sets him apart from Marth, Roy(from what I know) tends to play more aggressively than Marth. He's pretty much a glass cannon, since he doesn't really have the tools to play defensively (unlike Marth).
There's a
Reddit post that does a good job highlighting the differences between Marth and Roy. Also, once Apex 2014 videos start going up, I'd suggest you start watching some videos of Melee/P:M Marth and P:M Roy to see the differences yourself.
....of course, none of this will matter to you if all you care about is the fact that he's a clone.