From a purely Oz perspective...
We got rid of 1 and 2 c pieces ages ago, and everything was rounded to the nearest 5...not because of inflation and devaluimng of the currency, just that 1 and 2c copper were too expensive (and I used to get 2 lollies for a cent when I was a kid)...it now works pretty OK...pay cash, 1 and 2 round down, 3 and 4 round up.
Got dollar coins, and 2 dollar coins, and at the time people collected unreleased 10 packs of 1 and 2 dollar notes.
Coins are useful, came back from a buck's night the other day, and had $12 in change.
I would welcome a $5 coin in Oz, maybe not a $10...Why ?
Govt mint makes the coins, and sells them at a profit (face value minus metal value), no interest on them...
As to the collectors value thing mentioned before, they made the 5c, 10c, 20c scaleable in terms of weight, and the 50c worth maybe 30, then added special designs to the 50s, encouraging hoarding (we do, our kids do), and profiting basically 20c(ish) out of every hoarded "special" coin...got no problem with that, I can still buy 50c worth of stuff with them.
The $1T coin is an awesome extension of both the concept of making money on the face value of the coin versus metal value, AND the collectible side of things...who but the Fed would be able to say that they have the special $1T limited edition coin ?
...well at least for a couple of months, then we'd all have them, like my Zimbabwe note.