I actually made a post about the ending some time ago, explaining why it absolutely had to end the way it did, and that any other ending would have been a much deeper betrayal of the characters than their death.
Before I get to this point, though, I would like to say that Elend's and Vin's deaths were not suicides in the usual sense of the word. Technically it may be true, but the word carries connotations that I don't think really apply. Suicide implies that the death was somehow senseless or driven by an internal, emotional need to die. In a cosmic sense, Elend and Vin died in battle in order to save the rest of their troops. They knew it would kill them, yes, but if they could have found a way around their deaths, they would have taken it. But they couldn't, so instead of backing out or being cowards, they faced it with all the courage they could muster.
I think this is important because they had asked so many other people to die for their cause. "Redshirts" is a good way of putting it. Vin and Elend had to prove their core integrity, that all the terrible things they had done to others over the series were justified; that they were not tyrants or monsters but simply people trying to do their best. What better way to show it than to sacrifice themselves, rather than others, when that was what their cause truly brought them to?