Usually I assume siege units are more or less cruise missiles, at least for the first 1-2 hits on a city, and plan on replacing them with each battle.
I actually designed a wholly different approach to siege units in my personal RevDCM mod that addressed the whole 'suiciding catapults' issue, though it was much worse in vanilla civ than DW in my experience so far so I hadn't felt the need to propose any changes there.
In a nutshell, I limited the rounds of combat to a finite number (instead of swapping blows until the attacker dies or defender is brought to the combat limit), where a number of factors related to the defender's mobility (is the defender held down in a city, or in the open field, difference in movement points, terrain bonuses, etc.) determined by probability the number of first strikes the siege would get. So a siege unit attacking infantry in a city might expect to have 4 first strikes out of 6 combat rounds (on average, could be higher or lower), meaning only two chances (on average) to damage the attacking siege unit.
By contrast, attacking a highly mobile unit in the field, that same siege unit might only expect 2 first strikes out of 6 combat rounds, so much greater risk of being killed or at least taking more damage.
At the same time, the damage caused by the siege unit fell with the defender's hit points, so if you'd do 20 damage to a full health enemy, you're only doing maybe 12 damage when he's at half his health. This meant that attacking units already damaged would be less effective. This same principle applied to collateral damage as well, and is based on the idea that siege units work best when there is a lot to shoot at

, but in a practical sense meant you could never really count on knocking your enemy down to 10% of his strength. Usually a determined and overwhelming siege assault might leave most enemies at around 30-50% health.
The total result is a system where siege units where much better balanced with the rest of the combat system. By this I mean you could expect them to actually survive a few battles without having to be two eras ahead of your enemies, but they also weren't uber weapons, there was a limit to how much they could do by themselves.
Anyway, back to what we're actually putting in for now...
As for the general 0.5 minimum, I'm happy to give it a try, as I said. My suspicions could end up totally off base in an actual game. Another thought I just had is in regards to defensive victories. If the XP they give is globally half of what you get offensively, would defenders end up getting too little XP with a 0.5 minimum? Half XP on defense has always rankled me on principle, don't know how this would come out. Again, I'm happy to test at the lower value and see how it goes.
If you're in such a superior defensive position that the enemy has no business attacking you, then yes, you'll get very little xp out of it. And for justifiable reasons I think. However, by using fractional xp, we can make smaller upward adjustments on how much xp you get from defending as opposed to attacking. Before, attack xp (as set in the units xml) was defined as 4 and defense xp as 2 for all but hornets (where both were 4). Now it's 40 and 20, where those represent 4.0 and 2.0 respectively. We can therefore for instance change the normal defense xp number to 2.5 so it's slightly more than half of the xp for a similar attack, if it proves necessary.
f siege withdrawal could be folded into increased risk/increased reward, that would be great
I'm definitely looking at this, the main problem with siege in my opinion is the whole combat limit idea. As techs advance, this number generally goes up, meaning the siege units can do more damage,
but.... it also means they'll die more often at the same combat ratios as a unit with a lower combat limit because they have to do more damage before they pull out. Since the normal way of figuring xp from a battle is the essentially the combat ratio of the two units, it means higher combat limits increase the risk of each attack at the same combat ratios, but the xp would be the same despite the increased risk. I'll have to come up with a balanced numerical approach that gives appropriate rewards for the risk taken, and that will take some thought.
Chris