If you look at commercially made fountain pens, I have yet to see one with a spring. Unlike a Rollerball where the writing apparatus is free to move in the pen body, a fountain pen has the writing apparatus screwed to the pen body. Thus in a Rollerball kit, the spring helps mitigate any tolerance stackup in the parts to keep the nib properly exposed, while there is no such stackup to be concerned with in a Fountain Pen.
With my commercial Fountain Pens, (Aurora, Lamy, Namiki, Parker, Sailor, Schaeffer and Waterman) I always have to apply considerable force to pull the converter off of the nib assembly. No amount of ratteling around in my suitcase, briefcase, backpack has ever made a converter come loose.