Ah-ha! I meant to write "insurance on the rookies and free agents" as well, but somehow that slipped by my fingers.
I totally agree, though. I think it should be left up to the teams. Even better than a fixed roster size, why not give teams a sort of training camp salary cap, under which they can bring in as many players as they can fit within the cap? That would reward good stewardship of resources, though some might object it would allow scouts to be lazier by substituting quantity for quality.
For that matter, I don't really agree with the 53-man roster. We have a salary cap. Why not let teams do whatever they want with it? If they want to field a highly paid 44-man roster or a lower paid 65-man roster, why not let them?