I think we need to ask: just what are we trying to accomplish?
As best I can tell, when it comes to lighting in shower areas, we have three issues to address: personnel shock hazards, water spray hitting the bulb, and moisture inside the fixture. (Ironically, the advent of LED and other 'cool' lights might make that third issue a greater concern).
Personnel shock is easy: If there's any way someone can touch the fixture while showering, then we need to GFI it.
Shower trims are intended to keep water drops from hitting a hot bulb, causing it to shatte. Broken glass in a shower is not something I want to have to worry about. So, if there's any way for water spray to hit the hot bulb, we need to protect it.
Since most showers are set in something of a recess, ventilation is an issue. Water can, conceiveably, condense atop the inside of the fixture, then drip onto the hot bulb. Again, this risk is reduced by using a sealed trim.
The downside to completely sealing a fixture is that moisture that enters - and moisture WILL enter - becomes trapped. I have seen fixtures with over an inch of water that, over time, has condensed within in. More important, that moisture goes to work, causing corrosion and rust.
High ceiling? Open ceiling? Severely sloped ceiling? It's perfectly possible for a fixture to be located far enough above, or otherwise positioned, so that a shower trim is, IMO, not required. Keeping in mind my earlier comments about moisture accumulation, a shower trim might actually create a hazard.
LED's, CFL's, atc., may allow more moisture to accumulate, as they do not generate as much heat- heat that can keep the insides dry.
A final 'safety' issue is: how do you change the bulb? Remember, a shower trim blocks access to the bulb; if you can't safely set up a ladder, maybe it's best to leave the can 'open' so you can reach the bulb with a pole.
