On many SUOC trips, people use their Eos backups as their primary lights, and their big primaries with battery packs as backups. They run out of batteries quicker, but people find them perfectly satisfactory. They don't last as long as most lights with battery packs (like the Corona, non-waterproof Apex, HP10, DUO, and so forth).
I dislike the Eos as a primary light because the beam is too narrowly focused for me. But most people find it to be well-balanced. Furthermore, up here in the northeast, most cave passage is small enough that an unfocused, wide-area beam, like that provided by the Quad, lets you see the walls clearly. In the rest of the country, that might not be the case.
Of the lights I've used, the
Quad is the least expensive light that I'd feel good falling back on in a cave. On
Inner Mountain Outfitters, the Quad is $30 and the Eos is $38. (I'm not saying these are the lowest prices--that's just for comparison.)