I find this whole debate really strange, people want to use a model that doesn't have official rules and isn't produced by the company that releases the rules and the product range.
This is equivilent to wanting to use warmachine models in warhammer 40k or pokemon cards in magic.
I personally wouldn't want my opponent fielding something against me that isn't official at a tournament because it won't have been playtested by FFG who are in charge of game balancing and controlling the overall impact of a unit on the game. FFG CEO never said you can't play them for fun between friends but not officially, so they are on the same grounds as home-brewed stuff - ask your opponent first.
Finally lets look at two of the more materialistic points -
1st - why would you allow another company (even one you work with) to create and release products for a line you own/pay licence for and potentially release units with better rules so people will buy them and not your stuff? This point is not about the models it's the rules. Releasing control of your IP is a really bad idea.
2nd - FFG make this game. FFG products only. Simple rule. They do so much work to get interesting and balanced units out every month whilst trying to avoid new units warping the game or power creep (i'll admit power creep is going on though) why would you let someone else come in and potentially change that?
Nobody has said you can't field unofficial mechs in unofficial games but there have to be limits. FFG are a massive company it has to be one rule for all otherwise where do you draw the line, what if they said the dust games mechs are ok and another company comes along making tanks for dust? are they allowed?
Just my opinion but I do think some people are moaning about this without looking at the bigger picture of what's good for the game.
finally I leave you with the thought - maybe some of the stuff that is in the process of dust games having made and ffg playtesting might show up in dust warfare? and be legal from book release. it's what used to happen with 40k codex's when Andy Chambers ran GW - rules for things that didn't have official models yet.