Well, let's put it like this: the wrong reason for a manager to cut a 1:1 to biweekly is because they don't have enough time in their calendar to support their directs. A better reason would be because there isn't enough time between 1:1s to fill out the weekly meetings.

But the manager isn't the person who can easily make that judgement call. IMO, the correct person to judge that is the direct -- they can't just show up at their managers desk and demand a 30 minute meeting when something important shows up, whereas a manager can (and often does!)