What could be the advantage of a reduced internal shell in a squid?

to be more flexible to get away from prey

gladius

Yes, they have; although it lacks of phragmocone.

Their ancestors had internal shells, but now all the squid has left is a chitin-like substance called a "pen," and is used as a flexible back bone.

Their "shell" is actually a thin rod that runs the length of their bodies on the inside and stiffens them.