I was looking at a few mobile suits, and model kits and things, when I thought, why are Athrun's Mobile suits the only ones with the extra blue sensor horn thingy on their heads? Aegis has it, Justice, Saviour and Infinte Justice. What does that blue thing do? If it helps hime in battle then why don't any other mobile suits have it? I can understand if no other mobile suits in Seed have it (because the Gundam's are new designs) but surely by the time of Destiny other designer would have copied it if it was useful?

I think it's just the design, not necessarily "useful" in any special way. Have you noticed that all of Athrun's Gundams look kinda similar? They're all red, and the design of the heads look similar. I think it's just to identify Athrun as the pilot. Much like how the Freedom and the Strike look kinda similar in colours (blue white red) and the head antennas look similar, to identify Kira.

I guess so. It just seems more unique than any of the other difinitive things mobile suits have. I mean the Legend also has double head antennas and things. Maybe they're hinting that Athrun is more important? Or maybe its got to do with his self-destruct tendancy

nah^_^'
i think its just the design
like MapleRose said