For me the language is a mere tool to understand human beings. So in a sense I would say that the more speakers a language has then more is the reason to learn it - at least that's the case for me.
1. English
2. Mandarin
3. Hindustani (a combination of Hindi, Urdu and other North Indian dialects)
4. Spanish
5. Arabic
With these five languages alone, you can talk to half the world. Although I have always preferred quality over quantity but in this particular case I believe that quality emerges from quantity.