Sniper's high individual damage is useful in the early-game, but other Monkeys do more DPT for less cost and delay, namely the Dart Monkey Twins, and Mortar Monkey which can attack every turn. Use its shots against high-HP Bloons, and avoid overkilling a Bloon if possible.
While a decent candidate for Try this!, and serviceable in newbie decks, other Monkeys simply do better than Sniper's minor one-shot strength. Its only redeemable factor is being a Common, which increases chances of drawing a Monkey card.
The Leeching Shot Sniper, despite being an Uncommon instead of Common, has more utility than the extra damage per shot offered by the ordinary Sniper, making the former more valuable overall.
Unlike all other Commons, a Sniper Monkey can one-shot a Golden Bloon, even on Dark Castle. Also can one-shot a Swarm Green Bloon.