I take Singulair and Zyrtec together. My asthma is mostly triggered by allergies, though. Singulair helpes me not only with my asthma, but also it helps A LOT with my other allergies. I don't really have any side effects, but I've also been taking it for about 3-4 years. I suggest you listen to your doctor and keep taking it, because I don't know how severe your asthma is, and would never tell you to stop taking it, because, well, hey, I'm not a doctor
But generally, I personally don't think that you would have an asthma attack RIGHT when you stopped taking it. It takes a few days to leave your system, and your allergies would slowly get bad again, as would your asthma. If you take Singulair, as you probably know, you have to take it everyday, and it maintains your asthma. You can't take it only as needed, so maybe that's why your doctor is saying you can't get off of it. If it is causing you side effects that are unmanageable though, maybe you should try talking to your doc again about switching to something else. The good thing about Singulair though is that it's not a steroid.