The problem is that the plex used for alliance tournament can be refunded if not used. For example, I bid 5000 PLEX (50 in old terms) in the silent auction. If I am not picked, it is returned. If returned as red plex, it sits in wallet making an advantage for one player out of alliance funds. If CCP has to convert red to gold, it could be scammed in some way causing problems. And if you have to pay in gold PLEX, that would create a negative image for the alliance tournament.
Additionally, why do you not want to allow an alliance to buy plex in Jita and then offer them for sale in their home system? Under your method, only plex bought with cash would enable this, and that seems wrong.
As for why I want it allowed, 4 reasons… Eve has an open market economy. Limiting PLEX adds a limit to the free market. I do not want that added because it could become a stepping stone to other limited items. I do not want to see skins and apparel which may only be initially generated by PLEX purchase, turned into one sale items as well.
Secondly, PLEX right now are a simple item in the database. It’s no different than ammo. You can stack and the database records a stack of X, reducing the load on the database. Under your system, each plex must be tracked as a unique item to ensure it transitions at the right time. This adds burden and requires back end programming effort I would rather see spent else where.
Prizes… If I hold an event, I want to fund the event in game. I could not offer PLEX prizes as I could only transfer gold ones, requiring I spend money. That is a limitation. Yes I could just use isk, but I can’t make a stack of isk as a tradeable item without involving a trusted 3rd party. A plex I could put in a corp wallet to show it is there. I could even split the stack, jump into jita and be a loot pinata if I wish for a game wide event. I can’t do that with isk easily.
Lastly, I do not want plex people have spent money on and plan to sell but may be out of game for 6 months because of real life returning to being only able to sell half the amount. Even if a year warning is given, it could screw people over which I would prefer not to see.