Programmers are like Software30 Aug 2015
If you are a programmer you have to treat yourself as you do your software. You need to constantly patch yourself with new updates of added skills to be able to accept more client requests.
Being super productive on a single skill is never bad but knowing a single language or proficient in a single tool will couple you to a single role. The result is you will catch less fish this way. (Its just a metaphor, I don't regard my clients as fish by the way :)
Some local recruiters in Botswana might frown upon a programmer who can juggle many things at once. They will joke and say: Jack of all trades, master of none. In the rest of the world, these programmers are known as Full-stack developers.
One day a client/employer will say '..great, how about mobile development?' and you will have to say NO given your only background resonates from desktop apps.
However, There’s no shame in saying “I’m not sure how to do that,” or “Let me research that a bit before I say yes.”