It's just that I don't really like the idea to calculate the exact time each time I see a posting, a message, an attack or anything like that. The timestamps in the game and forum are most of the times used to get to know when something happened, like "the attack happened 2 hours ago". I don't need them to tell me what GMT time it was.
If you talk about time you still need to calculate. If someone tells you that you meet at 10 GMT tomorrow, you still need to find out when it is in your time zone. So that's not really an argument in my opinion. You definately have to differ the timestamps and the time you arrange a "meeting". To which time zone you refer when talking is completely different from what is shown in the game.
Just think of an announcement that an event will happen tomorrow at 9 GMT. The first thing that is normally done is calculating the exact time in the own time zone by substracting or adding the difference, like 9 + 2 = 11 GMT+2 tomorrow. To really use the "standard ingame time" for calculating, you need to calculate the hours left from current GMT time because this is the actual ingame time. Afterwards you add them to your current time, which is much more complicated and no one really does it that way. Otherwise the ingame time doesn't matter and everyone already sticks to the first option.
As alternative the game could provide both times. This happens in other games without a problem and works pretty well. I play another international game which is totally based upon meeting at specific times and planning for a week. Until now this never caused a problem so that someone didn't know when it should happen.