s/LaSH said:
In theory the jaffa wear that armour all the time. In practice they almost never deploy their helmets from the big old collars, perhaps because they look so top-heavy and the jaffa have quickly learned that tau'ri will ruthlessly capitalise on that weakness by shooting them a lot... goa'uld arms and armour are intended to look cool, not be efficient in times of war. (I think they explained that in the episode entitled The Warrior.)
You do, however, see Serpent Guard helmets relatively often on the show, so they haven't forgotten them altogether.
So there's a possible in-setting explanation...
I have caught a few of the episodes. Enough that this theory makes sense. The Goa'uld wish for the Jaffa and all others to look upon them as gods. A lot of their armor and technology is designed to be flashy, cool, and intimidating. Better to keep the masses cowed.
They are also arrogant and resistant to innovation, perhaps their genetic memory contributes to that. A mature Goa'uld already knows a lot about his race and their technology innately without having to learn it.
Humans have the ability to adapt and change more than any other alien race, which is one reason they kick so much ass even though their tech is slightly more primitive.
I have seen lots of reruns and read episode summaries from different fan sites so I'm starting to get a good picture of the different races. Most of the advanced races like the Goa'uld and the Asgard have stagnated in their development.
See the episode about the Goa'uld Imhotep. Even though O'neill showed the rebel Jaffa and Imhotep the superiority of human machine guns, they still preferred their light staffs (I don't know the name of it).
And the Asgard inability to defeat the replicators is similar. The SG teams have destroyed and defeated the replicators but the Asgard cannot do so. In fact, the Asgard believe that humanity is the next great race to follow in the footsteps of the Ancients who originally created the stargates.
Anyway, I was curious about the expandable armor though because I haven't seen it much in the reruns and the fan sites I have looked at don't explain it much.