Ok here we go...
The advantage of Arcade mode is that you can change the power and weight. Which gives you the option of putting more weight on the faster drivers and less on the slower drivers, or instead of weight, you could change the power if you wanted.
Also in Arcade, you arn't limited to Racing Medium Tyres. So in say a qualifiy hot-lap situation, you can load up super-softs. Then when race time comes, you can actually have a statergy if the races are long enough to warrant a pit stop or two.
The last few advantages of arcade are that, A) You only have to win one and you've got all the colours, B) It's alot easier to just pick a race on the track you want and set the number of laps. In GT Mode, all the FGT events are well over 90 minutes long.
In GT mode, you can use ballast to even out the field if you wish. Set-Up is also another plus. Personally, in arcade mode I find the FGT a tad hard to drive, but in GT mode you can fix this.
In much prefer the idea of the countries then just the boring old teams, it brings something new to the table. I understand that you might have 6 people from one country and 1 from another. However, you could have multiple teams from one country (e.g Australia A, Australia Gold, Australia Roo) and, or you could just take the top 2 times from that country and use them in the points (An idea I don't really like, but it works). There is really nothing stopping someone for racing for another country either if they need to fill the numbers. You could use the qualifier to set-up the teams, if you want the two fastest in the country in the one team, or the fastest and the slowest or whatever.
Anyway, just my two cents.