Is the term Power User still a useful term on resumes?
No, and it never was. Describing yourself as a power user is problematic for two reasons:
Anyone can claim that he's a power user because it's pretty much impossible to prove. It's on par with describing yourself as "goal-oriented", a "problem-solver", or having a strong work ethic. Not only are these phrases meaningless on their own, it's typically assumed that they apply to you!
As Alison Green says:
Your resume is for experience and accomplishments only. It’s not the place for subjective traits that anyone could claim without evidence.
Power user is a distinguishing term in that it seperates someone from an "average" user. As a result it means different things in different situations. In a standard office environment an Excel power user is someone who can create a pivot table without succumbing to a fit of rage. In a university research department that level of experience wouldn't even get you in the door.
Because it's such a relative concept, it's also largely meaningless: it doesn't actually add anything positive to your profile by being on your resume. Even if you are objectively a power user compared to your would-be colleagues, so what? They obviously don't need to be that computer-literate to do their jobs so what will you accomplish with your computer skills? If you can help Janice from accounting out with her mouse problem or explain how she should archive her emails then all you've done is saved the IT department a bit of work at the cost of your own time. It's a collegial thing to do but it doesn't make you a better employee.
The one area where this would help is in improving business processes, such as in departments that haven't kept up with the times. It can be beneficial to get someone with comparatively advanced knowledge of Excel, Access or even Word templates to automate some processes. But if that's the case you wouldn't just say "power user" on your resume. You'd point to past accomplishments where you actually improved processes and lightened workloads thanks to skills that you possess! It's called showing your work and that's what a resume is for.
Or as Alison Green said in a different post:
Many job-seekers just load up their resumes with [subjective] words, which is incredibly ineffective. Self-assessments from relative strangers count for basically nothing in hiring (and probably in life, too). I mean, I could proclaim that I’m brilliant and enormously charismatic, but you’d be right to be skeptical.
Instead, the key is to find ways to show that you have those traits. Employers want to see actual evidence of those things, not just proclamations. And the way you provide that evidence is to talk about what you’ve done that illustrates your work ethic, or your written communication skills, or your initiative, or whatever is that you’re trying to demonstrate.