Straight from the cable manufacturer's website:
The cable is used in conjunction with the Redpark Serial Cable SDK. Together these tools enable hobbyist, education and enterprise developers to write iOS apps that communicate with serial devices. These apps may be deployed for private use at home, at school or in an office. Under current Apple policy this cable may not be used with apps sold on the App Store.
From what I understand, Apple's MFI approval grants you the right to use the "Made For iPod" or "Made For iPhone" logos on your product, and you also get developer support from Apple.
These cables are already MFI approved and sold with a mark-up to indicate so. Your choice in using this cable is solely up to you, just don't expect to be able to use any of Apple's trademarks when advertising your product (if you do intend to sell something eventually). If this is for your own personal project/hacking, then anything goes.
There are some examples out there of people using the audio jack to send/receive data from their i-devices as well, which may be an interesting (and somewhat portable) alternative.
