Yes, many of them do come with a DC adapter, but that is just one more
wire, and with the computer already plugged in (my battery lasts about
30 minutes, now), that will make for a mess. I ended up buying the
new OnCourse with SiRF Star III WAAS enabled. The spare battery was
only $20. THat's not too much for the convenience and peace of mind.

I was flying with a friend who had a full Jepp system, all plugged
in. Shortly after take-off for a full day's trip, we noticed the
battery LED flashing on his computer. The adapter was faulty. Well,
we had to shut down and rely on only batteries when we needed the
system on. Two 12-hour batteries are much better than one DC adapter.