The book "Kuby Immunology" states that normal infected cells might present antigenic peptides on their surface via MHC class I, but these can't activate naive CD8 cells. Only infected professional antigen presenting cells have the necessary co=stimulatory signals to do so. This is why cross presentation happens. But then why does the process of antigen presentation on normal infected cells happen then, if it is of no use?
It is of use. They're saying that naive T cells cannot be activated by normal cells because the latter lack the appropriate co-stimulus, namely B7. However, activated CD8+ T cells can recognize antigens presented by normal cells and form a response. This is how cytotoxic T cells are able to eliminate transformed or infected cells.