The answer for me would be (if I had a shaper trying to do this of course) is I can easily create a cutter from HSS. I don't have a large 60 degree dovetail cutter for the mill. But I don't have a shaper so I'd have to buy a dovetail cutter and use the mill.
Or would I?
Having said that I do have, and they are fairly cheap, a set of three fly cutters bought at Busy Bee Tools ages ago.
Buy Fly Cutter 1/2" 3pc Set at Busy Bee Tools
Fly Cutter SetTop QualityCounter balancedMade of hardened steelUses standard tool-bitsShank Dia.: 1/2"Includes Head Dia.: 3/4", 1 1/8", 1 1/2"Standard Tool Bit Size: 3/16", 1/4", 5/16"
www.busybeetools.com
I guess once could create a 60 degree tool for one of those. The depth of cut is 0.383" so likely a 3/8" piece of HSS would just work although it would have to be ground down to 5/16" to fit. Or 1/2" brazed to a piece of steel and then hardened again.
But again, if one has a shaper why not use it since that's the type of thing it was meant to do.