The benefit depends on the schedules. I flew to MIA teice last december. First time with SAS immigration took less then 20minutes. Second time with ba and immigration took over 2.5 HOURS!!!!

Personally IF I am connecting in the US I prefer preclearance but of not then not a big gain

I think then ARN preclearance would result in more US flights for sure which would be great. The lounges at ARN are not ones I would care to visit anyways