Right. I always thought that what may have been going on in Ripley's mind before going the other way is "What if the flame thrower doesn't work?" Since it was all hypothesis and they probably can't trust anything Ash told them, if it doesn't work after it rounds the corner, she's dead. So she ran. I'm sure if she was cornered, she would use it but I really think, even though it was their only viable weapon, it was a last ditch weapon because it was only a guess.
And more to the point, Ripley was on the clock and didn't have time to get into a fight with the ALIEN. It was shuttle or shut off the self destruct.