I'm still a relative noob at Fantasy (9 games and counting under my belt, started at 8th release), but I'm far from sold on Seaguard. They seem way to expensive for spearmen with bows. I've been slightly underwhelmed by Archer shooting and Seaguard have an even shorter range.
You could do the horde thing with Spearmen for a lot cheaper. I definately would not take a horde of 50 Seaguard. One dwellers below and you just lost 325+ points.
I've also noticed that Seaguard make me play more defensively then I do when I have a mix of Spearmen and Archers. With Seaguard I stand back and shoot while with Spearmen I march and charge. When I sit back and shoot, my T3 5+ save elves who cost as much as some other armies elites get murdered by artillary, damage spells, or characteristic test spells. Oddly enough, being in melee seems to be a safer place for our elves.
Once in melee Seaguard are just more expensive spearmen. So with Seaguard you either sit back and shoot and deal with the nasty I mentioned above, or you move them into melee and get 1 maybe 2 shots off with the unit all game (no volley fire either if you move) for 4 points more a model.
I'm thinking that a good use for Seaguard might be as empire style detachments of 10 (maybe 15) on the flanks. Obviously you don't get the cool special rules for detachments, but a unit of 20 spearmen marching forward with a unit of 10 seaguard on the side for flank assistance and shooting on the move could be useful and won't break the points bank.
Just my opinion.