I've got a 1986-87 Maryland chartbook that also covers the Chesapeake.
I wouldn't use it for actual navigation, but I'm wondering if it is still at all worth keeping for armchair planning.
Thoughts?
Sure- you can go online and look at the latest charts and see what changes have been made. In many areas, little changes for LONG stretches of time.
Here's a site I use for online charts-
http://ocsdata.ncd.noaa.gov/OnLineViewer/
Remember that most chart updates affect vessels with 40 foot drafts alot more than 4 footers. As long as you keep in mind that you're using old charts and navigate with appropriate caution, they are a great way to have charts of that little harbor you might want to duck into in a storm without maintaining a full inventory of charts aboard...
A good way to stay on top of significant changes in your home cruising area is through the weekly Local Notices to Mariners. They are available from the USCG at http://www.navcen.uscg.gov/LNM/default.htm
In total they're a bit dry, but the Advance Notices and General sections are the most useful for rec boaters.
I don't see any reason you couldn't use the book for planning purposes. I'd recommend that you check any routes you plan using it against updated charts once enroute. :)
If you have electronic charts, keep it as a backup. Or keep it as a backup.