It really depends on your usage, and forgetfulness.
I have two, first ones side buttons stopped working so MS sent me a replacement stating it could be nothing but a hardware issue (didn't want me to return it, just told me to bin it), pulled the old one out of the drawer a week later and it worked as new. It's still sat in the drawer though. Unlike an argument for a second charger, because of it's size, plugging in etc, with a pen, well i can't really find an argument as to why you can't just carry your pen around wherever the surface goes.