Your question brings up a few points that I want to clarify, ranging from philosophical to rather technical. I'll try to keep it as brief as possible.
Suspensions are a tool available to moderators that allow them to resolve disruptions with the least amount of intervention possible. While the system recommends a length at which they might suspend a user, the choice is left to the moderator. The only guidance that we provide is to select the amount of time they deem appropriate to cause the behavior to stop; the length of a suspension in no way needs to be commensurate with the actual infraction it's designed to address.
The ugliest of arguments can, in many cases, be addressed with a 24 hour suspension being issued to all participants. We also make it clear that we're not trying to punish anyone, we just want to ensure that all involved have ample time to cool off.
Lengthier suspensions are common only when multiple suspensions have been issued previously. Even in those cases, the increased length is simply moderators trying to find the right amount of time for you to be away from the site in order for the behavior to stop. When that starts getting longer and longer, and there's no sign that a user will once again sit down to play chess instead of throwing the pieces around the park - it's time for them to leave.
Moderators have a special reason they can select when deleting an account:
This user is no longer welcome to participate on the site
They must also give a reason for this, a short explanation in the same form. Once initiated, our abuse mitigation systems begin tracking things and said systems are quite good at preventing users that have gone so sour that all they want to do is cause disruptions from doing so.
There's also another caveat to suspensions - if you request account deletion during a suspension and return to the site within the period you were supposed to be suspended, the system will re-issue the suspension in a manner that causes you to wait until the original suspension would have lifted. These anti-recidivism measures were put in place precisely to curtail folks that just kept recycling accounts to get around restrictions.
Now, if this is somehow tied to multiple accounts that the system didn't manage to piece together on its own, we can and will merge them all together, if those accounts are continuing to be problematic. That is to say, if you were suspended four times, and then one day showed up and just started contributing valuable things to the site in a way that didn't annoy anyone - I'd say the minimal amount of intervention was whatever it took for that to happen.
In short, in all but the rarest of exceptions, when the behavior stops, so does any grudge we once held against it. If, in the course of causing disruptions, a user did something we just consider categorically unforgivable (you can't really describe it, you just know it when you see it, a death threat would be one example) - then we would just continue to destroy their accounts as we found them and ramp up the abuse mitigation layer on affected sites.
It's hard to give a blanket policy because the only thing mods can do is evaluate each user as a unique case, and do what they feel is the minimum amount necessary. But, the information I gave above is a pretty good insight into how those decisions would be made. Each mod is different as well, and they are (so long as the majority of their community and fellow moderators support them) free to moderate their sites as they see fit. There's also the question of escalation, if we (Stack Exchange) have to get involved, then we also become part of that decision making process.
Wait, I think I remembered saying something about being brief, sorry about that.