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My quick answer is "only when the number of topics being used is unknown". For example, in the navigation stack where you use parameters to control which sensor topics are subscribed to in order to detect obstacles.

At all other times I think remapping should be used.

However, if there are cases where remapping is not powerful enough, then parameters might be appropriate, but I would say that's a work around and instead we should try to improve remapping to allow for more complex cases. For example, in ROS 2 we described how you could use regex-like behavior in remapping, e.g. remap /front/*/image_raw to /front_new/\1/image_raw (see https://design.ros2.org/articles/static_remapping.html#replacement-part-of-a-rule), which more cases than were allowed in ROS 1.