I realize this sounds harsh, but I really don't see how they add any benefit to the game that needed to be filled.
There is at least one very specific benefit they fill; one that is very important and Battles not only fill the need, but do so in a brilliant and nearly perfect way. Battles address power creep and have the potential to be able to adjust the game's pacing and flow. Essentially, flipping a Battle functions similarly to giving your opponent 3-7 extra life depending on the Battle. This means, that, as WotC prints Battles into the game, they're also printing a slight slow down of the game as well.
Battles are exactly what they claim to be, a "battle" that can, but doesn't necessarily,
turn the tide in the overall war. Ideally, a good Battle would be good but not great to play without flipping, challenging to
defeat, and then massively rewarding once defeated. There's also the potential for Battles that you defend which give a bonus until defeated, or that must be attacked until defeated and give you a bonus after they're defeated. There are so many cool design spaces for Battles, but even with just sieges, they are really fun in limited, make for much more interesting games in casual formats, and represent the possibility of something that could actually even help more established and competitive formats to have more game diversity. I think they didn't want to go too hard on the initial run of Battles, but they'll likely return to them soon, with something a little different and maybe a few more pushed ideas until they start becoming relevant in Modern or even Legacy, but they wanted to avoid altering the game too much on their first
time out, which is good, since that shows they're learning from pushing the Lorwyn planeswalkers a little too hard (though no one really plays them any more either).
As for your complaint about having to know 4 different mechanics (Undercity, Ringbearers, Battle Mechanics, and Contraptions), you seem to have forgotten about Riot and
Provoke and Affinity and Retrace and
Suspend and Bushido and Dredge and Delve and Scry and Flying and Goad and
Channel and Haste and Ward and Trample and Prowess and Adapt and Exert and Manifest and
Unearth and Madness and dozens more. There are hundreds of mechanics that you need to know to play the game, but most, if you don't know, someone will fill you in. Sure, I get Fading and
Vanishing mixed up all the time and it's confusing, but if it doesn't explain it on the card, I can always look it up. Knowing how a siege works is not any more of a problem than me playing a dragon with Amplify in my Ur Dragon deck. Most of the time, I just explain it and we move on, just like any card that my table's not familiar with. Don't equate having to explain how a card works with too much complexity. That would count out like 90% of the cards in the game for many players.