Is it worth the 6 bucks to get the Battle Tactics add-on since I already have TA and the Core Contingency add-on? The no-CD play might be nice to have, tho - I'm lugging around about 3 pounds of game CDs in my laptop bag, and GOG is slowly fixing that problem for me by offering more and more games from my aging collection. ; )

Get it.

I'd reccomend, even if only to complete the set.

Battle Tactics wasn't as impressive as Core Contingency, but it added some new units and maps, as well as a bunch of campaigns - though the campaigns are rather different... sometimes you don't even get construction options.
You do need it if you want to use any of the mods.
All in all, the CD-free option plus the additions above would be worth the $6 to me.