Is there a downside? Only if you’re the type of person who likes to nurse a drink for an hour. An "up" martini is meant to be enjoyed in about 10–15 minutes. You sip it, you appreciate its crystalline clarity, and you move on with your life.
What Does It Mean To Order A Martini Up at Indiana Margarot blog
But if you’re a sipper who likes a long, slow drink, go ahead and order it on the rocks. You do you. Just know that you’re accepting a trade-off: convenience for perfection.
Here’s a little tip for the brave: try ordering a Martini "up" with a twist of lemon instead of an olive. The citrus oils float on top and make the cold drink taste like winter air. It’s a revelation.