It's important to let a player win now and again. In a perfect world, there' no such thing as rage quits and everyone takes each defeat as an important lesson on how to improve themselves for next time. That's not really the case in the real world, though. A first impression means everything for video games, and you don't want to screw that up.
Warband's multiplayer can be very addicting because its combat is easy to use, but difficult to master. However, it's what keeps you playing the game. It's not what gets you playing in the first place. Besides, addictive combat alone can't be relied on to hold a player's interest, especially when it feels like they're just muddling along and hoping for the best. If you're jumping into a game for the first time and pick a siege battle, you aren't entirely sure what you're doing, everyone's kinda running around like headless chickens, and as you step up onto the battlements to see what's going on outside the castle you're suddenly chopped in half by a screaming naked man as he leaps off from a previously unseen ladder, spinning his greatsword around like it's going out of style. That's an easy way to suck you out of the experience. It's not exactly that players don't like getting beaten, and we can't let new player get their butts kicked. It's more like we don't want them feeling they're not welcome, or that it's not worth what would seem like a massive amount of energy and time to get better than they are presently in a game they're not even invested in.
There's a lot more to discovering and comprehending multiplayer than the melee portion of combat, too. Something as simple as figuring out how the interface for selecting your character, for starters. The cash-buy system, the differences between each faction and weapons, the way each map-type is played, getting used to the behavior of other players and how they function in a sparsely-communicates game, and learning how to work with your surroundings are all important aspects of multiplayer that would be better to learn in a less chaotic environment.