Except for a few tags, you need a significant investment to get marines (idea groups or 1000 ducat one per TC improvements). For that investment you could get far more manpower than you can utilize sailors via marines.
I suppose that it's true I wouldn't go out of my way to get marine forcelimit, but I do use marines with tags that gets them. Perhaps paradox could attach them to some more NIs and missions (Venice gets them from missions), as well as increase their percentage. Maritime and naval ideas need some help, they could be added there as well.