But these viruses don't have to become latent. The virus can then complete lytic cycle. Why would a positive sense ssRNA virus want to make it so complicated when it could just let the viral genome act like mRNA like other pos. If it strikes you as inefficient, it is. Lytic cycle has it's problems too.
Lytic cycle is a pain for viruses.
If you keep killing off your host, you have to find another, so sometimes it's nice to just rest somewhere for awhile. That's why latency or lysogency is good for viruses. It's a time bomb for the host cell but at least it doesn't kill it right away.