I've been disappointed with the damn things as well. I typically tap into magma with the ingenious plan of 'carve fortification, run for dear life, savescum in the event of failure'. I once had a fort where I swear the little buggers actually thought the area under my forges was a room carved especially for them. They never actually bothered me, however.
And benoit just answered as I was posting. That makes sense (as much as any DF bug does), much appreciated, though that does mean the problem's not exactly solvable.
A bit off-topic, but there is a better way to tap into magma pipes.
You know how the walls of the pipe vary in diameter?
What you do is find a level with a small diameter of the pipe, and then look at the level below that. There will most likely be an area where the bottom level will be larger, like this:
Level 1:
##############
#####~~~######
###~~~~~~#####
##~~~~~~~#####
###~~~~~~~####
####~~~~~#####
######~~~#####
#######~######
##############
Level 0:
#####~~#########
###~~~~~~~######
###~~~~~~~~#####
##~~~~~~~~~#####
###~~~~~~~~~####
####~~~~~~~#####
######~~~~~#####
#######~~~######
########~~######
########~#######
################
If you mine out a tunnel on the bottom layer like this:
Level 0:
#####~~#########
###~~~~~~~######
###~~~~~~~~#####
##~~~~~~~~~#####
###~~~~~~~~~####
####~~~~~~~#####
######~~~~~#####
#### ##~~~######
#### ###~~######
#### ###~#######
#### ###########
You can then channel out a space on the higher level like this:
Level 1:
##############
#####~~~######
###~~~~~~#####
##~~~~~~~#####
###~~~~~~~####
####~~~~~#####
#### #~~~#####
#######~######
##############
The hole that you channel will remove the wall on the lower level and give you a nice, safe flow.