Protection from being swallowed does seem an obvious explanation for
the various types of armament seen on gastropod shells, but - if that
is the reason for such projections then:
1.  Why are such projections so rare in bivalves?
2.  Why are spines and flutes found primarily on those bivalves that
are LEAST likely to be swallowed (sessile types like Spondylus,
massive types like Tridacna)?
3.  Why don't non-spiny gastropods living in the same habitat with
the spiny types get wiped out by predators through selective
predation (Murex capucinus among Murex cichoreus for example, or
Murex trunculus among Murex brandaris).  Do the "smoothies" have some
other, equally effective form of protection?
4.  Why are various modern gastropods (Cypraeidae, Strombidae, etc.)
LESS spiny than their extinct ancestors?
Too tired to proffer any possible answers today, so thought I'd just
toss a few more questions into the fray.
 
Paul M.