I wouldn't do it for two reasons:
1) My purist tendencies. These two species come from very different biotopes. Bettas live in a world of great stillness in places where they may be the only fish within whatever body of water they inhabit. These bodies of water are mostly floodplains, rice paddies, small swamps, ponds, very slow moving creeks, etc. The water is somewhat stagnant hence the development of the labyrinth organ. Zebra danios come from briskly flowing rivers and streams that have good amounts of space and lots of constantly flowing fresh water. These are two very different places. Equals two very different fish. Bettas would find the zebras annoying and unnerving - bettas are very relaxed like old retired folks and the last thing they want to deal with is that rowdy group of idiot teenagers who just keep driving past their house repeatedly and blasting their young whipper snapper rock and roll music.
2) Danios are nippy little buggers. So more than just driving by, some of the zebra danios will take to destroying some of the betta's property as they drive by. So now the betta is annoyed by being forced to live with rowdies who have no business living in his neighborhood, he is also being physically harassed by them. No fun for the betta!
Now while there is more often than not very little danger to the large scale health of either species, why would you want to annoy a poor innocent betta with a bunch of upstart zebra danios getting on his nerves all day and night? Just something to think about.