I just bought a "closeout" 4pc 8wt fly rod, and noticed that it did not come with rubber plugs for the male ferrules. Are those plugs necessary, from a structural standpoint?

I don't think so, though I put a bit of my Rod Bond that's left over from installing the reel seat and grip in all the male ends to keep out debris and give a finished look.

Sort of. A tube is pretty sturdy, except at the ends where the openings are. Flexing at that point can cause minute cracks that get worse over time. It's not imperative that you have those plugs, but it's easy to make them with Rod Bond or a little cone of firm EVA. Take a piece of brass tubing and punch out something just a little larger than the opening, compress, insert and glue in place and you're set to go.

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George,
Whenever I have a two piece blank and is in need of an end plug, I simply make one out of a solid piece of graphite or fiberglass.
I will take a piece of solid blank and chuck it into my drill. Then, using a desk top belt sander, I will sand down the blank so that it is a nice fit into the end of the blank.
A bit of 5 minute epoxy will secure the plug.
Then, I use my cut off wheel to trim the plug flush with the end of the blank. No worry about having the end of the blank crush in the other portion of the blank under stress.

Take care
Roger

Thanks, guys!