If a particular candidate, who happens to fail part of the interview, still seems like a good choice, by all means look for additional means of evaluation.
I once had an interview, where I spoke with a few people very positively for about 3 hours, it was going great. There was no sign of a technical test, so I had assumed by this point that they either didn't do them, or they'd be in a second round of interviews. Then they realised they'd forgotten to give me the technical test, which they then gave me and I screwed up for whatever reasons. Upon learning that I'd aced every part of the interview except the programming test, I mentioned they had an employee who'd worked with me on previous projects and could vouch for my coding ability. But they said they couldn't use that information because it would be unfair to other candidates.
This was a long time ago, and I didn't really lose much by not getting the job, but it always felt like the approach they took was wrong.