Pokemon Go is actually an amazing example of why it wouldn't work for the type of MMO that PokeMMO is.
Due to irl issues I've missed Treecko Community Day. The next day I've asked random dude if he has any shiny treecko for a trade, he nodded, we exchanged our friend codes to make it cheaper days later, and we agreed to trade treecko for some random dratini. He had like 20 treecko, so they became almost worthless to him, but he needed another normal random "iv" dratini.
I gave away like 13 shiny swinubs to people, because >swinub and I had like 26 anyway. Who needs 26 shiny swinubs, honestly.
Now, imagine the same situation in PokeMMO.
Next Saturday we announce that there will be shiny Tauros weekend. Well, you just have spent 100+m on shiny tauros today, and you are literally screwed since your shiny will become basically worthless, or at least VERY reduced in value. This can potentially kill basically the whole point of trading shinies- I wouldn't trade for any shiny, if I would know they can come in "PokeMMO Community Day" one day.
This also applies to the "motivation" aspect you mentioned- sure it will increase shiny hunt on those days, but will also damage hunt on other days. I wouldn't go hunt for shiny treecko today, because what's the point, it has no increased rate, and I can get one easily from players, even if it will take few days to trade.