I have a similar setup to this, with a Thinkpad x230 and a budget 1600x900 HP monitor, and I also use dwm. The second monitor allows me to work with items that I don't need to interact with frequently.

For example, I can display the PDF output of a LaTeX document on one monitor while keeping vim and a PDF of a book I'm reading on the other monitor. If I only had one monitor, my productivity would suffer because I would have to constantly switch between windows.