Heh. Jenova was always in the background, a "puppetmaster" of sorts, but with no distinct "presence" in the game. A completely inhuman entity, virtually devoid of anything that a player could interpret or 'connect' with. Sephiroth provides a good means of personifying that evil, giving it a face and a unique identity, with virtually the same goals and values. It's like Exdeath taking human form in FFV, rather than having the party chase a tree around the world...
Jenova was more or less inert, at least the characters in the game appeared to think so. Gast and co. thought she was dead, which is a pretty reasonable assumption after millennia embedded in solid rock; Jenova never sought to break out of Shinra's confinement, even when they started extracting bits and pieces for experimentation purposes. (Of course, there's the additional theory that Jenova wanted those experiments to take place, and that even things like Mako reactors and the Wutai war were her doing, but I'll not go into that...) So basically, Jenova was either content to be used for years, or else was nearly devoid of active powers as a result of her treatment at the hands of the Cetra. That power naturally would return after being freed by Sephiroth and flung into the Lifestream, but even then it'd still be limited, presumably - beheading is often a bit of a setback, even for power-tripping alien busybodies.

Even then, Jenova's powers are apparently passive rather than active. Influencing minds, crafting illusions, leaving false trails. The only time Jenova's powers take an active role is when the illusory Sephiroth is around: the decision-making, the pontificating, the violence and bloodshed is all heralded by the actions of the being that looks, thinks, and reasons like Sephiroth. The final fights take place against beings with Sephiroth's approximate form. The fights against Jenova are quite telling: Birth, Life and Death comprise almost the entirety of Jenova's body, yet they never move or speak, only fighting like mindless things until they're destroyed - like they're running on instinct, perhaps a potent form of the Reunion instinct. The rest of Jenova is being lugged around in the form of Sephiroth. Then, the party reaches the North Cave, and it's time for Jenova's head to burst onto the scene: Jenova-SYNTHESIS, or perhaps one should say, Jenova-LEFTOVERS since that creature is little more than Jenova's head and a few extraneous chunks and tentacles. SYNTHESIS is clearly one of the most powerful opponents in the game, understandable since Jenova's head, and therefore her brain and mind, are at its core. However, the even more deadly foes are the ones that combine the powers of Sephiroth, Jenova and the Planet together into one - Bizarro Sephiroth and Safer Sephiroth.

Without the Sephiroth component, Jenova is consistently fallible and less effectual, throughout the game. Hence my belief that Sephiroth was a big part of things. We know that Sephiroth, not Jenova, used the Black Materia and called the Meteor, because the Materia was handed over to him. That act would make no sense unless he wanted to unleash its power and reap the rewards; otherwise Jenova simply would've got the orb delivered to her, and done the deed without her son's aid. Jenova clearly needed Sephiroth in some way, and for that reason he lead the charge and was at the forefront of their combined assault upon the world.

Hmm. Been a while since I posted anything that long-winded. *Searches for life*