Marc has not released a "Canon list" for MgT, nor for GT. Nor has he released a single unified canon list that I've seen - Each edition has a separate canon list on Marc's site.
Not to mention, T5 blithely writes over some bits of MgT and GT, and those two have incompatibilities. If they're both part of the same canon, then there's a huge wooly mess of canon contradictions.
Well I'd argue that there is an official canon, which is documented through the stories of the various ages of Traveller. The very start of T5 with the description of the universe and the various wars throughout Travellers history clearly links all the 'canon' Traveller together. As for third party companies like Gurps and MgT, well they have gone off on there own tangents putting out what they believe to be a good idea for Traveller, not necessarily fitting in with the core system as it was in terms of continuity, which CT,MT,TNE and T4 have all done from what I've read so far.
In the past third party companies seemed to support Traveller as opposed to invent there versions of it. (Eg, DGP and the fine FASA deck plans etc). Now there seems to be a trend in either making it the way the third party company would like it or simply throwing out the Traveller universe and creating there own universe based on Traveller rules.
For myself I am a firm believer in not re-inventing the wheel. Aliens modules have been done before, end of story. Well yes they have, but do they satisfy the current depth which like it or not, Traveller has. The universe has evolved into a massive gaming arena where characters could be dead before they reach the other side of the known galaxy. Races have been evolving along with humanity and the T5 book clearly shows all the terrific additions and enhancements to human characters that have come about(also gratefully received as CT kind of bugged me that your character essentially had no choice but to enlist in the military, now that's not the case and Traveller is finally opened up to a greater depth of gaming possibility in my firm opinion. I guess CT kept things simple, but it was by no means all encompassing. T5 on the other hand just about is). T4 was the past. the early days of travel amongst the stars. While T5 on the other hand is the future, years from CT,MT and TNE. I'm looking forward to the expanded depth and character generation possibilities that the major races can provide for Traveller in a way that's only been briefly touched upon by past alien books.
That's why I believe a T5 Aliens book or books are essential. They deserve to have the treatment that the human character creation process has been given. Also friends who enjoy having a go at playing alien characters are always wishing there was more information on them. Sure you can make it up, but that's what leads to all this mess with discontinuity etc. On the positive side a T5 aliens update on the major races can put a stop to that and at the same time provide an excellent spring point for minor races and the ultimate creation of more races. T4 was superb in it's attempt to bring more minor races to the Traveller universe. Those very races would have evolved an aweful lot in the time span between the milieu zero age and the modern T5 age surely?
Dare I say it, the previous issues between the major star faring races may very well be dissolving into a more co-operative galaxy? This is just one of the many exciting aspects that the much needed update could provide.
Finally on the subject of other Aliens books. First of all Gurps. Correct me if I am mistaken but straight up it's an alternative Traveller where the CT age never ended. Eg, there's no shattered imperium timeline or virus timeline.
So straight up, Gurps falls outside of 'canon'.
Then on MgT, even if it's considered canon by some, it's support material is all hinging around 1105 and centred in the spinward marches, which places it back in CT days. I suppose you could even label MgT as support material for CT. (Feel free to jump in here MgT fans I only have some of the supplements for MgT, finding it somewhat below TNE standards but that's just me).
So canon wise, Traveller stopped at T4(which was actually a prequel game really) and has only just continued with T5. The interesting aspect of Traveller D20 or T20 is that it seems to be between T4 and CT (again correct me if I am mistaken but that's what I'm reading).
All editions of Traveller are terrific(with some liking not liking others), myself I've enjoyed bits from all of them but what always draws me to a SciFi game is the possibilities for the future. Well finally we have it, T5, the future of Traveller and while it may not be the rules to end all rules compendium that most Travellers believed it would be(although I think it is so far), the fact remains it's a step in the direction of moving Traveller forward and what happens in the future? So the Aliens modules are deffinately needed as we do not have the updates for them in this new Traveller future.
The other main reason is exactly as I've been saying. The Aliens books are still not complete and what I've seen from Mongoose, is OK, but certainly comes no way near matching the voluminous wealth of information and detail that T5 is going for. So new Aliens books are definately required to fill this void.
If you enjoy the CT years, then I guess you don't need them, however if you enjoyed MT, TNE and T4, then it stands to reason your going to want new support material for T5 just like the other timeline areas of the Traveller universe. Yeah? There's a good deal of people who want to see the evolution and expansion of the Traveller universe.