Explorer is meant for exploring and they have a lot of skills that help with that. Marshdweller/Explorer has some nice perks and I've played the combo numerous times. Not the best combo, but not the worst.

Swarm of leeches is an amazingly awesome spell, which is why it's only available in swamps. Giving them the option to use it without edge purchase or with diminished capacity would be akin to giving a ranger a lesser rot spell.