Is it necessary to learn 3d?

[Piotr Jasielski]

It is not always necessary but 3D is becoming more and more important asset in the concept art industry, especially for environment artists. It's not as important for illustrators though.

I think you should probably try it, possibly learn some simple things, nothing advanced. Most concept artists do just basic blockouts and then paint over them.

I'd also recommend blender - it's free and a powerful weapon.