Additional to that is even with an alternate if the unpredictability of weather or poor forecasting models are the real issue then holding an alternate may not help you as the Mildura incident proved. Two airliners both holding an alternate got caught out at both the destination and the alternate.
Neville, I hear what you're saying, but the Mildura incident is not a good example. Although the two aircraft were carrying extra fuel, neither had
planned Mildura as an alternate for Adelaide. The TAF that was valid for their ETAs at Mildura required either 60 minutes holding or another alternate due to a TEMPO period for low cloud. The two aircraft chose to divert to Mildura on the basis of the Mildura METARs after the Adelaide weather closed in.