Was Merc: 2000 a good addition to the rules?
In my opinion yes. It added some new rules that weren't included in T2k such as noise levels, sleeping enemy garrisons and guard dogs and the Quick Kill Rule (which would show up in the 2.2 version of T2k but wasn't present in earlier editions)
It also included some extra equipment that was definitely useful for T2k such as the Individual Tactical Radio, Snorkel Gear, Extreme Cold Weather Gear, Grappling Hooks, Suppressors.
And probably the most important reason for me personally, I could never find enough people interested in playing a survival/post-apocalypse game so I could never get a group to play Twilight. But there was a lot of interest in mercenary type games because the crowd I started gaming with had played lots of Advanced Recon and MechWarrior so mercenaries where a "cool" subject to them.
So Merc was the game that allowed me to dive into the T2k 2nd Ed. rules and then later into Dark Conspiracy and Cadillacs & Dinosaurs.
Now that last game was important to me for other reasons, I loved the artwork done by Mark Schultz and was very vaguely aware of his Xenozoic Tales comics (Cadillacs & Dinosaurs being a licenced game of the comics)
Having the game motivated me to hunt down Xenozoic Tales so that I could actually read them and I'm still happy to have them in my possession.
If the T2k series had not included Merc, I might very well have missed Dark Conspiracy and Cadillacs & Dinosaurs. My T2k books would have ended up sitting on a shelf with me wistfully staring at them wishing I could get a group to play it
Does it deserve a reboot?
I think a reprint with updates to T2k 2.2/Traveller: TNE rules would work fine. I think there's still enough interest in small scale/tactical combat rpgs and mercenary/PMC type games.
I would hesitate to ask for a reboot because I generally don't think reboots do much for the original game. Often they change too many aspects and it becomes a totally new game.
As an example of what I mean, I have always loved the X-Com video games since I was first introduced to them on the NES. When the 2010 remake titled XCOM was announced I was initially very interested, then when I saw gameplay footage and borrowed a friend's copy to actually play, I was left bitterly disappointed - it was nothing like X-Com and should have been called something totally different as it missed almost every single thing that made X-Com great.
I feel the same way about the reboot of Twilight: 2000 by Free League (and enough has been said about that, so I'll leave it at that).
Generally, I don't think reboots work (with some exceptions in movies/novels)