I'm one of those professionals managing client accounts. And I have met many other professionals account managers. You'd be surprised how most of them do this.
Logging in and out of accounts is not a problem, not when you have a MCC account. But I have run into agencies who don't have one and some have never heard of it.
Adwords Editor is a great tool to work offline but I recently talked to a manager who did not know about it.
You are correct. In order to efficiently manage accounts, whether one or hundreds, software tools to help is a requirement. You need something powerful to analyze all the data in countless ways. Here too, I've run into agencies who just don't have the proper tools. Most seem to use Excel which is inadequate to truly reveal insights.
As for myself, I have a background in computer database programming. I have the tools. I know of no commercially available tools but then, having created my own, I haven't looked. My guess is if there are some available, they would not be desktop-based but SAAS.