Daniel Defense MK12 - Why an Exposed Gas Tube?

I have been looking at these SPR type rifles - why do they leave the gas tube exposed? I understand they are wanting to use a rifle length gas tube but why not just add a longer rail? Is there any reason that they are purposely doing this?

Is it an adjustable gas block? Sometimes they leave the gas block exposed so you can adjust the gas without removing the rail.

Do you need longer than a 12 rail? What would going from 12 to 15 add other than weigh? Also putting things that far out on the rail will mess up the balance.

No I'm just asking because part of the gas tune is exposed

That's how all MK12 rifles look. The armorers at the Navy probably only had rifle length rails at their disposal when the MK12 was created, which is why you have an exposed gas block. It is a non-issue, IMO.

If you want, swap out the rail for something longer and it'll cover the gas block which should make you happy.

that makes sense - just still see the rifles today from groups like Daniel Defense that are built the same way...