Is it necessary for an instructional designer to be a content expert?
There are instructional designers who specialise in a particular industry (e.g. energy, banking, pharmaceutical) or specific job (e.g. marketing, sales, management). Thus they do not rely on a Subject Matter Expert (SME) and save a lot of time since they are already familiar with the content. They have stories to tell and examples to share.
On the other hand, the problem with expertise is two-fold: you may no longer remember what was difficult when you were still a novice and you may be tempted to give everything you know to your learners (which is likely to be too much).
Instructional designers with no specialisation may find interesting connections or come up with innovative ideas. However they do need to make the effort to understand the content. If they don’t, learners won’t either.