Dominae, I understand your concern about the "slippery slope" issue. On the other hand, when you start worrying too much about slippery slopes, you're on a slippery slope to never doing anything for fear of starting down a slippery slope. Seriously, the best course is often somewhere in between the extremes, but "slippery slope" arguments tend to push people so hard in trying to stay away from one extreme that they end up at the opposite extreme.
The reason the GOTMs are such a mess is that the folks who run them have a deliberate desire to create things that have nothing to do with the standard game - and to go on creating from one month to the next. The AU Mod's philosophy is far more conservative; no one really wants a new unit and graphic, but a new unit is the only way to implement a changing level of ability for a unit and a new unit would be confusing if it doesn't look different. I won't say it's impossible that we might someday want to incorporate another graphical difference for similar reasons, but given how extreme a situation it's taken for us to want to do it once, I'm not too worried.