With the opaqueness thing, perhaps the shields only shield against specific frequency ranges. But given the overlap with lasers it seems that sometimes the shields would make it hard to see.
Star trek likes to use the polarity excuse. They're out of phase, so they pass through, they match the polarity so they deflect... yeah, great, but they don't address what happens to all of this new energy introduced into the system. Oh, inertial dampeners... of course...
Yeah, given masers (microwave lasers), xray lasers, etc, lots and lots of the EM frequency can be weaponized today - I rather doubt future capabilities will be less effective than today! :)
Start trek? Is that related to Star warts? :D
The other big gotcha with the question is that the EM spectrum is self-ignoring. You can cross 2 lasers and (in general) both will hit their targets with the same effects as if they didn't get crossed. (ignoring phase effects at the point of contact between the 2 beams, of course)
As far as Star Trek, don't forget the Heisenberg Compensators, not to mention the Tribble glued to the phaser!
Stargate: Universe uses the frequency-alternating mechanism to block laser fire on Destiny, explaining that higher frequencies would divert lower frequency EM, most likely what they meant was a higher concentration of photons. The science is a tad shoddy but, as you said, the suspension of belief make it geek-worthy.
I wouldn't know. That never happened.
Right. Assuming your "shield" is em, and the incoming energy is em, the two waves pass through each other. Sure, they create an interference pattern, but that doesn't do you any good sitting behind the emitter that's doing the interfering as the wave wouldn't be modified once it passed the emitter generating the interference.
I guess a better solution would be to simply have a bay where Tribble's are provided with food and then they're dumped into space on the side that needs to absorb the impact of whatever weapon...
An old game called Traveller had "sand-casters" which had that effect for spaceships, except they used sand rather than the 90+% efficient Tribble replication engine....