While for majority of cases the US citizen spouse would not be asked to participate in N-400 interview, it does happen in some cases. Even with I-751 approved. After all, citizenship under 3 year rule is based on marriage to US citizen. Even mere presence of US citizens spouse in the waiting area is another check box in IOs head confirming the marriage is going fine and reduces the chance of additional scrutiny.
I'd rather be safe than sorry.