I never even thought about it… I used my Odst disc for Mythic map pack 2. I guess I’ll have to pony that up. Oh well it’s been 3 years since I played and $5 will be worth it if it helps me forget the current game.
I’m in the same boat. I bet a lot of others are as well. I’m happy to pay the 4.99. I’m just glad to have a fully digital copy of Halo 3 on my box. I still have my original discs, but they are put up in storage. It is really convenient to have the digital copy, even having to pay 4.99 for the Mythic II pack.
While I understand that it’s slightly annoying that you’ve gotta buy the DLC, I give you my personal thanks for doing so. 
Couple years back, I had some spare Microsoft Points and decided to see what I could get from the Marketplace. Spotted the Halo 3 DLC and went ahead and purchased it (at full price might I add). It was only afterwards I realized that I owned the Multiplayer Disk that I got with ODST facepalm haha. Pretty cool that they put it on sale now though, hopefully more people will pick it up