In many ways, Sony's new memory format for Vita seems like a step backwards. The pricey cards are a proprietary format exclusively for Vita--an odd step when you consider the PSP used the slightly more open Memory Stick format. So why did Sony change its tone for the Vita? One developer cites security as a reason.
Sony Division 2 Software Development Head Muneki Shimada said that they needed a format with "guaranteed performance," as the read/write speeds of cards across multiple formats and publishers can vary. By sticking with the proprietary route, Sony could "ensure the security" of the platform.
The PSP was plagued by rampant piracy, so Sony's attempt at making a more secure memory format from the get-go seems understandable. (However, that doesn't make the pricing any more acceptable.)
Vita's memory card can't be used as "mass storage" on a PC either. Whereas the PSP let you connect it to any computer to freely add whatever files you wanted to it, you'll have to go through a proprietary content management program. This downloadable utility is required in order to manage the contents of the Vita memory card. (PS3 firmware 4.00 also adds a Vita-compatible file browser.)
Sony hopes that by having an intermediary program, managing information on the Vita will not only be more secure, but easier to use. "You do not need to remember which folder contains data on the memory card," Shimada told Impress Watch (via Andriasang). The PSP used a rather cumbersome folder structure, which made using the PSP as a media device especially cumbersome.
Perhaps one of the few bright sides to Sony's switch to a proprietary media format is the addition of Mac support. While you could use a PSP as a mass storage device on a Mac, Sony never released an Apple version of its Media Manager software, meaning access to the PlayStation Store was extremely limited for Mac owners. Vita's Content Management Software Assistant will not only be available on PS3 and Windows, but a Mac version will also be available.