Well the higher voltage preouts will allow you to keep the amp gains lower, and provide better S/N. Like I said, this might be an option for an audiophile package, because like you said you have no problems with the preouts now, and they are definitely good enough for most people, but then there are those that want the hi voltage. As for the divx I don't know how much cpu power it needs, but that may infact be a problem, all I know is that vcd and svcd are on portable mp3/cd players, so if divx doesn't need much more power then it shouldn't be a problem. I do agree with you it is a bit outrageous, but it's for the passengers, if you look in any car audio magazine, the biggest fad now is multimedia, and most cars have tv's in the headrests for back seat passengers, and they even have video game consoles in the back. So that's why I would suggest that if it's not a hard upgrade it might be an idea. I plan on purchasing a dvd player, so in my case I would definitely use it.
Of course this is all just my opinion too :)