Reasons:
1. Node.js is maturing and growing by the day
2. You want to invest in something that becomes better [more reliable, mature, documented etc.] over time, and the Node.js community is going to be great over the next few years
3. You don't want to invest in the technology of the previous decade, you want to invest in the technology of the next decade. Node.js is that technology.
4. Building on Node.js lets you have an advantage in hiring because developers that know Node.js tend to be very good hackers [they were curious enough to learn something new].
5. My company, Flotype, is taking Node.js to enterprise with our NowJS offering. Every single major corporation out there, such as Zynga, Google, HP etc. are all working on Node.js right now. Enterprises are looking into the future and are betting on Node.js.
Honestly, there are benefits of sticking with something like Rails, but you should be looking into the future, and the future is Node.js