I paint everything I do anymore with base clear just because it's easier, actually more forgiving, and I'm not a pro. I have used a lot of acrylic enamel and had some good paint jobs with it. The base coat dries much quicker so if you're painting metallic, it's not as easy to get mottling. It's also easier to fix a mistake in the base before you put the clear on. If you screw up the clear, it's also fixable. The guy who's painting your truck probably knows what he's doing and won't have the typical problems I do. He may also be looking at the fact that acrylic enamel is cheaper. Don't let him talk you into synthetic enamel, it's still cheaper but I did a pickup with it once for a guy and his horses ate it off.

'65 C-10, 65 L-60, 65 C-80, 72 C-60 & and a bunch of old Chevy cars.
Never time to do it right, always time to do it over.