A reason to have separate accounts is that not only do you terminate access, you also have an easier time ensuring that less of what that person had access to could have been compromised. (This of course goes right out the window if said person has sudo/su access, in which case you have a much harder time, but even then giving them individual accounts means your opportunity to audit becomes so much greater)
After all, it's not the honest guy who'll never try to log in again you're primarily trying to protect against (in fact: for the honest people, a good security policy protects them by making them less likely to become potential suspects if/when something happens - it's in your own interest when you leave an organisation to ensure you get locked out), but the guy who might decide to try to do something later, or who might even be thinking about doing something before they leave.
I haven't, but if you assume actually malicious users you're probably going to end up with something so locked down it's useless. Aren't you?
Now, you must also have a functioning system, and so you may take risks by leaving things more open than you would like if you don't have the resources to thoroughly lock everything down.
But wherever locking things down further costs you very little, you should take the opportunity. And elsewhere you should asses what level of protection you can afford. Ultimately it is a cost-benefit analysis. Many risks are not worth spending money protecting against. Others are vital.
But even disregarding malicious users: Individual user accounts is not just a protection against malicious users, but against careless users. When someone sets a password that gets guessed, you want to be in a position where exploiting that persons credentials is as hard as possible, and tracking down actions taken via the account is as easy as possible.
And yes, you could insert something into a build script. But if the build script is committed, and the commit was pushed from a named, individual account, you're now at the risk of going to jail. Creating deterrents is often a sufficient risk mitigation strategy to be acceptable.
A developer is more likely to create better and more easily maintainable software if the target audience is assumed to be an ordinary user with no special system privileges. In my experience, when a developer has root and assumes everyone else does, deployment becomes a nightmare.
What I was trying to say was that there's not really any way for you (server admin guy) to know if I (software dev guy) have inserted something malicious into a script that all the other software folks run constantly (software build system, NOT server build/init script, NOT deployment script).
This is not about the end-user's privileges, or server set up, just how in a team-base software dev environment you're probably going to have to have a measure of trust for your employees.
Note that as an attacker, there's a high risk of exposure and identification in the scenario you describe, and that's a good thing. A well secured system shouldn't merely prevent attacks, it should also protect innocent users from suspicion (another reason why shared accounts are discouraged).