Let's think for a second about this card. It costs one green mana and one colorless mana, and it's a 2/2 creature in itself (because it gives all slivers, including itself, +1/+1). That makes it very reasonable to use by itself.
If you're playing with creatures at all -- there's a good reason to use this card. It's good by itself, but with other Slivers it gives both strength and protection.
Combos with other cards:
It's obvious how this helps other Slivers. A Sliver Queen generates cheap high-value slivers really well with this. The bonus applies to virtual cards as well. Why this card was a Common in its original release -- I don't understand. It doesn't have the cachet of Lotus Petal from the same set, but use a card which adds or changes an attribute to make one's other creatures into Slivers and it's clearly more valuable.
Ways to counteract it:
Play a sliver deck yourself, or try to destroy all creatures.
Cheap. Boosts other cards.
Dies against creatureless/control decks (though your opponent might not understand how strong it is).