All of the disadvantageous genes are discarded. But gene exchange allows a complete variety of genes to be reacquired from their original sources of diversity. If however, gene exchange is too limited, genes which are lost cannot be reacquired easily, and then the whole diversity of options must be carried through each generation, as occurs with phenotypic variation. The cumulative effect of gene exchange is adaptation by improved function. The cumulative effect of phenotypic variation is adaptation by multiple options.
Improved function is more adequate and complex than multiple options.
So the phenotypic variation of Morchella is a disadvantageous necessity resulting from the use of ascospores and a carry-over from its yeast ancestry.