Playing cricket with 8 players is never fun. having spent last season with most games having 8, it always bugged me how the opposition captain wanted to bat first. it was clear they was going to win and smash us around with the gaps in the field. but this form of posting a massive score which a team of 8 players is not going to get just destroys the game. this often puts the younger players into a bad mindset about the game and may not want to play games. At many of our levels cricket is played for enjoyment as for many of us our county days are far behind us.
On the flip side, if you are against 8 players and you bowl first, and they only post a small ish total (say 150-170) then with the gaps in the field it's less of a game for them. I'd imagine that and the fact people can see 50/100's being easier than normal means they bat first. I have to admit I'd bat first and be aiming for a 50/100 just for personal stats. LIke you say, the win is obvious (assuming it's a similar level of players!).
I know that's probably the most fun for the team with 8 but I'd imagine most would do this as they would enjoy it more.