I have a Scubapro Mako (titanium, blunt, straight blade) affixed to my BCD. It weighs almost nothing. The attachment points are secure. It is always there. I've never been asked to removed it, even when diving in certain parks that forbid them.
I dive with the Spyderco Salt Assist. Small, compact, folds blade in so no exposed sharp edges, and has both saw and blade cut.