I read this as, "adding too many characters can dilute the story and putting in chars that ARE NOT important, people won't care." And if you add chars that aren't important, well...

IMO, DD did that a lot with the comic. Just throw people in for no reason other than a cameo. Cameos are fine but if the story starts to revolved around them, then those chars have to be used again and again.

Imagine if Spirit's lil adventure trying to find CC ended up being Dart a few pages later. And then they had Dusty and Red Spot. Then since Beach head hasn't been used for 2 issues, better throw him in. Oh, if you have Beach head, you gotta have Gung Ho. And if you have Gung Ho you have to have Bazooka, Alpine and... You get the idea.

Now I think about it... I think that's the reason I like the concept of the Greenshirts. You can take a few select Joes and get the reader to care about them. In a big battle, you can use the GSs as fodder and it won't matter if they die or not. Easier than keeping track of what 100+ chars would actually be doing.