I assume most bullion is acquired because of the known weight of the silver or gold content, which makes it easy to sell the coin for its melt value. But, what about Proof bullion coins? Do they exist more for their collectible value than their melt value? If yes, should they be graded and certified?

That's up to you. Certified Proof will increase the value of the coin depending on condition. Also grading regular MS will increase the value again based on the raw condition. Allot of fees. Check with the ANA if your a yn they have a deal with NGC. It's up to you. The most important thing is the coin will be protected. Otherwise it will lose grades over time. I wouldn't even think of melting unless that's all you want to do. I would start with 100 gram bar. And done by a professional with the right paper work. I'm not buying a brick unless I know where it came from. And what's in it. It's work.

Proof bullion coins are collector versions and certainly can be stabbed if that is your wont.

If your asking as an investment, both uncirculated and proof versions bear the same melt value. From a numismatic stand point the proof versions general tend to bring higher amounts than uncirculated. In the market place they always carry a premium well above melt value, and being graded increases that further. Often tremendously so  in the highly sought after MS/PF70 grade of eighties gold commemoratives. 

Why not? There is more bullion coins out there. Good bullion silver bullion. There both precious metals. And a slabed bullion coin gets more in the market place. Otherwise you have a 34.00 coin in a box.

I have some certified silver bullion from other countries by NGC and PCGS but in "gem uncirculated" grade (which is somewhat generic) and doesn't mean much, save for the fact the specimen in question is genuine, which is whats important to me.