I still think reliability is the key issue of the fire prism. Vypers with brightlances are cheap enough that you can afford redundancy. Fire prisms are too costly to do this. Not necesarily too costly in terms of points, but costly in terms of force org slots. Between falcons, reapers, war walkers, wraithlords and even support batteries, the eldar have too many other awesome heavy support options. If you were to fill two of those slots with prisms, just to get a degree of redundancy and reliability, I think you'd be denying yourself access to some of the best stuff the Eldar can put on the field.
The other issue is that it's only a small blast. That means if you target troops, your enemy has probably spaced them well enough that you'll only get 1 hit and maybe 2 partials at best. If you fire at troops then, you almost definitely won't get your points back (assuming you hit an average of 3 times in a game, and still have to roll to wound). At least you will insta kill most models that you do wound, assuming they don't have crazy invulnerable saves. If you attack vehicles you may do better but you still have a good chance of failing to penetrate armor or only generating a crew shaken result. And it's already been mentioned that against any truly tough armor, brightlances are a better choice anyway. If you're using it against vehicles then the blast radius has gone to waste. At that point, a falcon's D3 S8 pulse laser shots(to say nothing of its other armament) seem more reliable for tank hunting than a fire prism's single S9 shot.
Maybe if you played on an 8' by 8' table... then the range would make it useful enough to try.
Make it a large blast, and up the points by 30 or so and you'd have a much more interesting heavy support option.
Disclaimer: I'm about as green as you can get, so feel free to disregard/scoff at anything below based on that fact.
Most of the improvement suggestions I've read fall into two camps: improving the sketchy reliability (which you've enumerated quite nicely) or giving the Prism Cannon a bigger bang to make sure the enemy really feels it when a shot hits home. The first set of fixes are the more prosaic of the bunch - increased BS for the tank, having the cannon act as if twin-linked, a BS bonus when using a Targeting Matrix, or whatever. This also seems like the safer approach, as well - there are a lot fewer ways to increase reliability, and the effects are generally better known and playtested.
Balancing the big risk of lousy marksmanship with the reward of a bigger kaboom at the receiving end might be more "in character" for 4th edition, though, given the "big risks vs. big rewards" mantra that Games Workshop seems so fond of. It's also more entertaining ground for speculation - ordnance templates, combined hits from multiple units, etc. However, this is more likely to result in 2nd-edition-style rules creep and has a less predictable impact on game balance, particularly if the "reward" bit has to be bumped quite a bit to make up for big risks, high point cost, consumption of a valuable Force Organization slot, or what have you. Plus, I'm not sure units with really high risk quotients make it into peoples' forces, regardless of the payoff...
The lack of definition regarding the Fire Prism's role doesn't help, either, since it's hard to make a change that is simple, balanced, and applies to all of those ill-defined roles. My guess would be that we'll see GW slot the Fire Prism into a "vicious tank killer" or "ruthless troop harvester" role and fit the Prism Cannon's abilities to that role at the cost of its abilities in the other. But we shall see.
So, enough with the rational consideration. As someone who was motivated to finally play after years of only painting by the Fire Prism characterization in Dawn Of War (however ashamed I am to admit it,) I'd like to see some kind of interesting nastiness added to the cannon. How about tracing the path of the beam to a vehicle/terrain bit/board edge and laying a small blast template on every unit intersected? Or throwing stuff around by causing everything that survives the hit to scatter from its original location, possibly getting thrown off the board, out of coherency, or destroyed by movement into nasty terrain? If it makes a solid hit a "woohoo!" sort of event, I might even have to start playing with Eldar instead of just painting random models because they look cool.
