Looking up information about ZFS SLOG, I always see the same advice: "SMB uses async writes and thus ZIL provices no benefits." It's sane advice as ZIL brings no benefit to asynchronous writes. Sane, but no longer true.
Samba 4.7.0 changed default value for strict sync parameter from no to yes. Practical consequence of this change is that ZIL SLOG will be useful even for CIFS shares and adding it will bring you some benefits.
Should you add ZIL SLOG is still question highly dependent on your actual clients and how your pool is structured. But answer is definitely not a straight "no."