I find the notion of a challenging AI somewhat ridiculous. To me, AIs are just simple patterns you need to learn - and then victory is inevitable. Not really something that excites me
Pretty much depends on how you play.
E.g. in 4X games I never ever reload, because otherwise, yeah, victory is inevitable in most cases.
While I also mostly play such games in Multiplayer, I am mostly playing together with just one or two other players. Having a couple of AIs adds some additional dynamic to the mix and makes the whole game more "alive".
With that I mean that while I see how a 1v1 player vs player experience can be fun, playing 2 out of 16 nations or so on a huge map in Civ 4 is an extremely different experience which makes you much more immersed into the "world" if you can call it like that. A little bit like Europa Universalis or Hearts of Iron.
That said, I love how they implemented multiplayer in GalCiv3 (or at least from the concept as I haven't played it yet).
To explain it to people who aren't aware of the situation:
Most 4X games have a big problem of taking turns in multiplayer. You have the choice of going full turn based, or going simultaneous.
Full Turn Based has the advantage that you can take your time (within the timer if you set one) and that it is completely strategic. Your PCs speed or your ability to click fast have almost no impact. It is in generall the fairest way to play. With the exception if the game puts a lot of pressure towards the beginning or ending of the turn (e.g. if you are the last one to take a turn, you might have an advantage in planning production in cities, players whose turn is already past, cannot for example change production to military when this last player destroys all their units). But this is highly depending on the rest of the game mechanics but hardly is a factor from what I can tell.
The big disadvantage: It just takes AGES. And with each new player one turn takes longer.
Simultaneous Turns: All players start their turn at the same time and can do actions at the same time. This menas that a player who is faster might get some benefits by that.
Lets say that two scouts are at a hut and could get it in that turn. The player who can click there faster, either due to loading time or due to faster reaction, will get the hut.
Another example would be the following:
Player A shots at Player Bs melee unit with an archer.
Before Player A can shoot again with a second archer, Player B moves the unit away.
Another example: Player A has to movement points and gets near an enemy unit and discoveres it, before the player can click to move away again with the second movement point, player B attacks him with his unity (or the city) which might kill player As unit.
So generally playing simultaneous turns is a pain in the ass if you want to play strategy instead of twitch based RTS. But it is necessary if you want to play a game in a timely manner and not via email or something like that.
Personally I used to play
Civ 5 with my friends (still have the scores of 25 Civ:BE and ~100 Civ 5 matches)
with some house rules to actually make it work.
While Civ 5 offers to switch dynamically to turn based when war is declared that makes it take ages. So what we did is doing everythind simultaneously BUT troop movement once some players are at war. E.g. Player A declares war to B, C is still at peace. Player A goes into the boarders of B, and B can decide either to react in this turn or to move first in the following turn. Either way, we basically change to turn based combat and tell in TS when the troop movement is finished so that it's the next players turn. However all the players can do all the other actions, like city management, moving troops in the backyard withouth having to wait. This basically gave the advantages of both worlds.
And now we finally come to
GalCiv3's System. While GalCiv 2 did not include Multiplayer at all, GalCiv 3 implemented it in one of the best possible ways: It basically does what we already did via houseroules, just in a little different way. All players cann always give orders to planets, and also give orders to units. However during the "Master Turn", there are multiple sub turns of each player where these movement orders are executed.
So lets say, that turn 1 starts.
Player 1's turn begins. He gives movement orders which are directly executed, he sets up his planets and so one. During the very same time, all other players set their constuction plans and already give orders to their ships.
Now Player 2's turn begins: His ships will automatically move the way he planned it during Player 1s turn. If they still have movement points left, he will be prompted to use them now.
And so on. It is an extremely simple, but extremely good solution and I am wondering why nobody else implemented it like that. (well, I guess one of the reason is, that Firaxis as the other big player doesn't care much about multiplayer and completely focuses on singleplayer which also explains all these horrible mechanics and balance flaws in Civ 5 which hurt MP but hardly affect SP).
Sorry that it got a little longer.