Sure you could fire up MonoDevelop, and do C# in Linux, but for me Mono was still a unknown factor. As the status of mono project had been very uncertain till Microsoft decided to support it officially. Second problem I had with Mono, was partial .NET Framework support, I do hail the efforts of Mono community, but lack of some of the features from .NET Framework made it harder for me to motivate myself to code seriously with Mono. But with new .NET Core I see no need in trying to work on Mono, when I know there is runtime that will work similarly on all the major platform.
On other hand, Visual Studio Code might be better choice for development on Linux.
Update: It was 1½ years ago I wrote this post, and the popularity of VSCode has sky rocketed since then. A lot of developers don't like to work with Microsoft Stack, but will gladly use VSCode from Microsoft. Today I am using Visual Studio for all
Unity, and VSCode for everything else. On that, this update was written in VSCode.