Shuhei Yoshida addressed the newly introduced requirement that a PlayStation Plus subscription would be necessary to play online on PlayStation 4 in the most recent Weekly Famitsu. As translated by Kotaku, Yoshida states that online play will be a primary pillar for the console and that the requirement was the only way to meet the growing costs that come with maintaining the service's quality.
Here's the full quote related to the PlayStation 4 and PlayStation Plus:
"The main pillar for the PS4 will be online play. We're developing many new ways to play and connect which requires a large investment of resources. Considering the cost, to try to keep such a service free and consequently lower the quality would be absurd. We decided that if that's the case, then it would be better to receive proper payment and continue to offer a good service."
From there he details that only one subscription will be required per console, so families or roommates can share without doubling down on price. Also of note is that PS+ won't be required for all online content on the system like Microsoft has done with Gold on the 360. PlayStation Plus is only necessary for "real-time online play" and that games or video services that don't meet that ... oddly vague term won't require PS+.
Sony's PlayStation 4 launches this holiday season for $399.