Banshee's Blade is an interesting Equipment.
The potential is wild. Everytime you deal
damage with a creature it's attached to, it gains
a counter. That includes defensive damage, which
is great. After a few combat phases, you can
have a "Build Your Own Monster" equipment.
It works nicely on flyers, as you are more
assured of dealing some damage with a flyer in
this environment. I could see it as a casual
constructed card, and it's definitely worth
considering in a sealed or draft deck.
I can't really think of a use for this card in
Constructed. It's really a
win more card. Rarely will it pull you out of a
tough spot because either
the creature you're putting this thing on is
winning, and therefore doesn't
need it, or it will be useless as the creature
will die in a few turns.
In Limited, it's a tad bit more useful because
it's a permanent pump to
your fatty. I wouldn't draft it highly but it
could be cool late game.
Imagine this card on a Clockwork Dragon.
In casual, lots of rogue, slow decks can come up
and it's in those decks
where this will succeed. It's also a lot of fun.