If fun counts as a legitimate civilian purpose, you're losing the distinction with tanks. They would be a blast to drive around and fire on a ranch, which is why Tony Buzbee had one. And that's pretty much Beto's point. Fun, hobby, curiosity, whatever, isn't enough justification for certain weapons given the cost.
The ban would be on owning AR-15s and AK-47s. We're talking about Beto's statements, right? That's what he proposes. And if the practical use for those weapons is culling feral hogs and slaughtering roving bands of meth heads when you don't live within 30-45 minutes of police, as you said, they don't seem very useful to me for most Americans. For the small number of Americans who do have those needs, there could easily be exceptions for areas without police, or for animal control if that really is the only effective strategy for hogs. Registration, licensing, limited use allowed, etc.
But after that tweet you mention went viral, and the "out of touch lib" backlash happened, I saw it reported that four people have been killed by feral hogs in the history of the United States. Four. They may well be terrifying, but I'd bet there are other ways of dealing with them that don't require our country being flooded with these weapons.
I hear you about rural Americans. But why are Dems the ones who always need to start listening to the concerns of rural Republicans? They haven't given a shit about our concerns on guns for years, as people have been slaughtered in urban areas across the country. Multiples more than deaths by feral hogs or groups of methheads too large to handle with a shotgun. At this point, yeah, my view is, fuck 'em. I used to care about the legitimate uses for these weapons, but now I just don't. They have their right to vote, and so do I and the 80%+ (and growing) of Americans who live in urban areas. My view is, we'll see who should really be listening to whom and trying to compromise.