Another thing to consider, Gen III is the first generation where distribution devices were either flat out made available to the public (Ageto Celebi disc, WISHMKR Jirachi disc, PAL Pokemon Channel, Eon Ticket e-Reader card) or were stolen from their event locations (the Shiny Zigzagoon GameCube demo discs, European Aurora Ticket cart, the MYSTRY Mew carts, the various 10th Anniversary carts). The same can't be said about the Gen 1 or 2 distribution methods. And personally, I wouldn't worry about the Gen 1 and 2 events (besides Virtual Console Mew) because unless you use a PC program, those Mews and Celebis will be forever stuck in Gens I and II.
For me, the fact that these Gen 3 events are able to be migrated to future generations is the reason to preserve not just the ones already collected, but the method to generate more. Especially considering some moves are still exclusive to these various events.
So bottom line, it's not about the actual distro ROMs (even though those would be AMAZING to have), it's about the method they use to generate the Pokemon or wirelessly send Wonder Cards so we can put that into custom ROMs that you could tell apart from the real ones.