I'm sit firmly in the "rigid" camp. A removable headshell on my titanium arm would have defeated the purpose.
The pro's & con's are mostly obvious as already stated ... however ... one MAJOR disadvantage of removable headshells; that's rarely addressed, is the rotational play between the headshell and arm isn't accounted for during attachment, causing azimuth errors, often severe.
I've often witnessed a change of cartridge w/removable headshell (at homes & in demos & shows) in which the author didn't relate/twist the stylus to achieve proper azimuth. Worse ... most didn't even care to try or were aware of the issue. Never mind small errors - often the cartridge is visibly offset. The sonic result is a very stressed and unfocussed sound, but instead of blaming their setup, they blame the cart. With line contact types, this is highly critical.
I'm consistently amazed at the number of times I've witnessed this happening by people claiming to be "knowledgable-vinylphiles". I once witnessed a Cart shootout demo using many expensive carts in which the author (a well respected audiophile) changed every cart without considering the obvious inconsistent azimuth per cart. I mentioned to him & his audience that his demo methodology was missing a very key element, and without that, the demo could be considered "useless" ... but like any self respecting audiophile with an audience ... he proceeded anyway ... (sigh)
Ironic, because, if properly realized & understood, the fact that the removable headshell can be rotated to attain proper azimuth means that you can set azimuth on any cart relatively easily.
tb1