Because Utah paved the Legal Tender way with any gold or silver coin minted by the federal government (not including coinage by other governments) that's what I stick to.
Since most people who are taking delivery of physical metals are buying bullion rather than coin, I can envision the possibility later on, with our crafty little out-clevering and maneuvering federal legislature (that I wouldn't put anything past), a future scenario wherein they officially acknowledge gold and silver coinage as money, as Utah did, but continue to treat bullion (and stocks and other paper derivatives) separately, as a commodity investments (so that they can tentacle in some additional revenue from so-called "commodities" investors). I see that being rationalized on the basis that actual coinage would be a very small fraction of the actual metals in existence, and therefore not a serious loss of revenue. Check and mate, because the task of getting your metals converted into coinage is fully under the control of Congress and the U.S. Treasury, as everyone not familiar with the minting process gets a new word called "Seigniorage" added to their vocabulary. In other words, you can enter the "new currency"...after you pay your dues.
Since I would not put that past them, and since I am NOT "investing", nor "buying", but rather converting my crappy fiat currency into my specie of preference - I only accumulate gold and silver coin - only that which was minted by the federal government, and never with a view toward collectable or numismatic value of any kind. So very little bullion (like less than 1%) in my holdings, for that reason alone.
Now consider it in reverse: Is there some reason why junk coins could ever be singled out in a way that made them substantially less valuable, on the net, than bullion? Even if I could think of any reason, the process of converting coin into bullion is considerably different than the process of minting bullion into coinage.
Nicely stackable bars and polished rounds might look better to some, all Fort Knoxy and stuff, but since the difference in the difficulty or ability to exchange either on any scale is hardly substantial, for me it is an absolute no-brainer.