Silver doesn't get you much, early boarding, 25% mileage bonus and free upgrades on domestic flights (you'll be pretty low in the priority list, however) are about the only tangible benefits (full list of benefits here ). But it is better than nothing.

If it is easy for you to qualify (i.e. no extra expense for mileage runs) and it wouldn't interfere with your elite qualification on UA (or whatever other airline), then, sure, go for it.