As an ADC main, I can guarantee that it's 100% fine to leave your adc in lane. The biggest problem is knowing WHEN to do it. If your adc isn't doing to hot, it's perfectly fine to go and help other lanes, but only if you'll actually be able to help. Like some of the other people have said, you may not want to go to a lane if your mates don't want you to. You can even ask the others if they need/want assistance be it through vision, some pressure, or a full gank. (please at least tell your adc if you're doing this though. Otherwise we're just left to kinda think in you're a dick)
Certain matchups are really bad to leave solo, such as Kai'Sa or Twitch, as they'll just be hammered over and over again. Champs like Sivir or Lucian can be fine, however, since they have a form of wave clear or safety on their own. Leaving your adc can also help get them back in since they'll be getting solo xp, as well. By roaming around, you might even draw the enemy support to other lanes, leaving the 2 adcs alone in bot which is far less likely to be volatile and put your struggling adc further behind.
I played a ranked game earlier today, actually, where i was on sivir but had an especially rough laning phase vs a cait/morg. Shortly into the game, my thresh left lane and helped around the map. He got our mid and jg some kills within a few minutes and actually forced the morgana to try and match him in terms of pressure so I only had to deal with cait in lane. It's much easier to farm up and catch up when you only have 1 champ trying to beat you down. Everything went well from there, (aside from thresh trying to flame me) I ended up getting my items in a timely manner to help out my team by the mid/late game and went on to win it.
I think the biggest risk, however, is the fact that most adc mains, especcially in low elo, are very prone to tilting if their support leaves them. I learned pretty early on that losing is a part of winning. You can't win every lane, every match-up, or every fight, but you have to know how to lose gracefully and minimize casualties. That's something that a lot of people still don't get and so they'll just run it down and blame everyone else.
In the end, it really comes down to the team you have. If they are willing to be helped and if your adc is equipped enough to not die and farm up for fights.
I hoped this helped! Sorry that it kinda turned into a wall of text.