They have been around for atleast 30 years, probably longer, useually Apples and Pairs are the fruits of choice, althought I have heard opf it being done with citrus aswell. If the tree looks healthy I say go for it, although you shouldn't let the tree get too big because I'll bet the trunk can't handle that much wieght.

Gee, my Grandpa (who died in 1976) had a tree with at least a dozen apple varieties. There is no mystery about it; scions of various varieties of apples (or pears) are simply grafted onto the parent tree. Home Depot is making available an old idea that has come around again as fresh... and why not? Great for a small family to be able to harvest Yellow Transparents in Auguest, Macs in September and Spies in October/November. (Although I think it is even more fun to start with one's favourite apple tree and graft others onto it.)

Different verities on one citrus tree has been around for 60 years, that I know of, you just graft what ever you want on the stock, works fine for a home home grower, grooves are usually specific to a single type.

Yep. Nothing magical about multi-variety trees.
Some require a bit of work to keep in balance, as some varieties are much more vigorous growers than others on the same rootstock.
Biggest gripe I have is that you usually don't get to choose the varieties you'd personally prefer (I'm not a huge fan of Macs or Golden Delicious, but that's my own preferences) - and sometimes the selections that work well in, say a cool zone 5 setting, don't work well, if at all, in zone 7 or 8.

$45 sounds a bit steep, but I graft all my own trees, so maybe it's in line.

You know, you can graft the Top of a tomato plant to the roots of a potato and get bot products from one plant.