This card was replaced in Tenth Edition with a more powerful,
higher-cost version, the "Relentless Rats", but the Swarm of Rats has
some advantages. Costing only (1)(B), rather than (1)(B)(B), rats can
be stacked more quickly, for a higher total power. It also applies to any
rat-type creature, making it much more open to creativity than
Relentless Rats. The obvious down-side, is the poor toughness - Only
1, leaving it open to a great deal of red spells that could wipe out your
force very easily. Most decks that use this sort of thing are a cheap
trick - You might win very quickly and easily, but the gambit is flimsy. If
I were going to create a "rat" deck, I would look at fortifying it with
spells that defend, and/or fuel the force - Take white spells that buff all
your creatures until end of turn, for example. Such a spell would protect
against instant-red death. Any color might help - Green can instantly
buff creatures or make extra mana, blue can counter and draw extra
cards, and red can add a trample effect, or buff every rat's power. I
don't have a lot of experience with rat deck designs, and from what I
have seen, making them work will be difficult in competitive play - But,
one thing I noticed, was that my design was most effective when I
minimized the number of */1 rats (I was using the new version, "Rat
Colony", in Magic Arena). An over-simplified rat-deck is very unreliable,
and it is most wise to incorporate as many effects as you can.