....a couple of years ago the coolant replacement procedure had been amended in the service manuals for all model years to include the recommendation for the use of the conditioner. I checked bak in early 2007 and found this snippet from the SOA site for the MY2007 schedule:
To prevent cooling system leaks, always add Genuine Subaru Cooling System Conditioner (Part no. SOA635071) whenever the coolant is replaced.
....unless something has changed since that time.