They can take damage from monsters and traps, but monsters don't
always seem to target them (it may be random). Similarly, it seems to
be random as to who trips over the trap. So they aren't too useful as
damage sponges.
Similarly, they aren't particularly useful in combat. They have
pitiful hitpoints, don't seem to attack until the second round, and
almost never "hit" anyway.
They are most useful for sending back to the village to sell your gear
(to keep you from becoming overburdened with loot), although you can
only do this in certain areas (like those dead-end corridors with the
secret doors). Except you then keep the cash on your person, so if you
die you lose it all. Better to make the return trip in person so at
least you save your status.
Given the price of the henchmen (5-125 gp to locate one, and then
60-100gp to hire) /and/ that they only stay with you long enough for
one sortie in the pit, they definitely are not worth the cost.
If you have a "hi" charisma, you can parley and bribe monsters to act
as your henchmen. I had two kobolds on my team at one point. They work
under the same restrictions as the hirelings, but at least they don't
cost as much...
Still, this is all nitpicking. It's a freebie tossed to fans,
extremely clever and enjoyable to play, and a nice nostalgic flashback
to the early days of AD&D.