Everybody always talks about Corrosive Orchid's dissolve effect, but it is also a vine conjuration with an attack, which I consider to be the more important aspect of it (think about the power of Wizard's Tower or Temple of Light). When I cast it, I often don't bother with the dissolve effect at all. An extra conjuration-timed unavoidable attack with corrode can be extremely useful in many scenarios:
- Removing Voltaric Shield, Block, Forcefield tokens, and other mandatory defenses right before attacking with something else
- Attacking low-health flyers or creatures with defenses
- Comboing with push/pull effects like Thornlasher's snatch (or corroding armor right before a shove through a wall of thorns)
- Abusing creature suppression effects (conjurations are immune to the obelisk, idol of pestilence, and other such effects, allowing you to get around their limits). This is one reason my Druid deck runs Mordok's Obelisk - True, there aren't enough conjurations with attacks yet to have a pure "conjuration swarm", but between the orchid and the lotus you can supplement your creatures enough to have significantly more attack actions than your opponent.
On top of this, you have the one time dissolve effect - a nice bonus, but certainly not the main reason for running Corrosive Orchid.