Rhuarc
Explorer
Since when are Solo monsters bosses? I don't understand that idea at all.
They aren't per se. But seeing that the whole party should have a challenging fight ahead of them if they face a solo monster, it is just logical to assume that such powerful enemies aren't anything less than "bosses" in the campaign.
Of course it comes down to how the GM wants to use solos in his games, but most of us use them pretty rarely as such a fight should be a major event during the adventure, and not something the PCs have to deal with at every corner.
The simple combination of the power a solo monster possesses, and the rarity of how often the heroes will meet one of them just sparks the idea of a boss opponent for me and many other GMs.
Of course you can go ahead and throw Purple Worms, Berbalangs or other "not-very-bossy-solo-monsters" at the party, but for me those are exception to the rule that solo monsters are usually the feared, infamous, final enemy of an adventure or campaign chapter.
Such a fight should be the awesome finale and solo monsters are best fitting for it