I don't do it, mainly because I think it's often used as a cheap/lazy way of getting more spearmen.
I really dislike the cocktail sticks blu-tacked to the back thing and even when complete spears are glued on the back I hardly ever think they look good. In fact they normally look like exactly what they are, a spear glued to the back of a model, there's very rarely any attempt to convert a holster or strap or anything.
If I were ever to do it I'd try and get some bow and quivers to glue to the backs of my spearmen rather than the other way around.
it's worth pointing out that all of this comes from a visual/fluffy perspective rather than a tactical one. I'm primarily a visual player, 99% of the time I try to play with everything painted and I just never think the spears stuck on the back look right. I pay a lot of money for my figures and don't want to ruin the effect by sticking cocktail sticks etc. on the back. For me, any in-game advantage you get isn't worth the damage to the aesthetic of the figures, but that's just me. It's the reason my Groblog will always stay on his giant 'shoot me' rock (I've seen a couple at tournaments cut off and glued to the base) and my eagles are about 10 inches off the table in trees, they get shot like nobody's business but they look cool
Tactically, sure, why not, if you've got a few points left over then chucking a couple of spears onto archers is a good way to use them up.
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