One of the problems I have with phase shift is that the phased person can still see an unphased person. Surely they should only be able to see matter in their individual universe
Not necessarily. The way it's explained in "Time's Arrow", one of the universes is lagging behind the other. Our two protagonists in "The Next Phase" might be lagging behind Picard and the others in the fashion of Stephen King's Langoliers - seeing the trailing ends of the actions of the regular heroes, the lingering afterimages. But when they see those, it's already too late to respond. And in some fantastic scifi fashion, they aren't just late from a particular event, they are totally late for every millisecond of their new, phased existence.
That they aren't late from gravity (the artificial sort that keeps them on the floors) may tell us something about the nature of phasing - but it may be just a matter of degrees. The heroes of "Time's Arrow" didn't sink through the ground, either, but they also didn't have the power to walk through the walls. Perhaps with a bit more phasing, LaForge would have fallen through the gravity forces just as easily as he pushed through the EM forces that made the walls solid.
That Ro thought she might be dead may not have been too far off the mark, either. Assuming, that is, that a person in her profession is likely to face death from the barrel of a phase weapon... Again just a difference of degrees, with the ghosts of phasered people made so thin that not even fellow ghosts can see them.
If phasing without cloaking was possible it could also be useless, it seems to be relatively easy to bring things or people back into phase, when it happened to Geordi and Ro all they had to do was flood Ten Forward with totally harmless particles and they were back. I think it's very possible tnat all it takes to force an enemy ship back into phase would be a modified deflector dish.
When the phase-cloak was used tactically in "The
Pegasus", it seemed to have more penetrating power than in the "The Next Phase" scenario - no resistance at all from the thickness of rock, and no gravitic effects. Perhaps a matter of degrees again, with more protection against easy countermeasures simply because the knob was turned from 0.47 all the way to 11,000?
It can easily be tracked and followed by other ships, it may be intangible but presumably its weapons are useless too while phased.
But the weapons no doubt can be de-phased at the flick of a switch... And the phased intruder won't flick the switch until the weapons are deep inside the soft belly of the enemy, well past his armor.
So it just sits there and can't do anything, not even spying because every bit of information they gather while completely visible would be known by their enemies.
Why would that be an invalid mode of spying? "Ah, I see you are planning for your big secret offensive here. Never mind me, I'll just take notes and then go. Oh, you pack up and go as well? I'll come along, it's no bother, no, really, you don't even have to hold those thick armor doors open for me or anything. And I can see inside that briefcase and that pocket and the brassiere and... Oh, what an
interesting mind you have! Mind if I map your neurons while I'm in there?"
Timo Saloniemi