Why not LWC or Lockette?
Protecting single blocks is effective versus theft, but not versus build/break grief. Sure the bad guys can't steal your stuff, but they can knock down your walls and pour lava over your bed! Both LWC and Lockette must be combined with other anti-grief plugins to effectively prevent build-based grief. GriefPrevention effectively protects against both theft and vandalism.
Managing permissions for single blocks is also rather tedious. For each block you want to share with a friend, you have to use a slash command or edit a sign. That's okay for a couple of blocks and a couple of friends, but imagine trying to create a community build (workshop, anyone?). GriefPrevention allows you to modify permissions for all the blocks in an area with one command.
Both LWC and Lockette can be used to grief. For example, a griefer can surround your chest and block your doorways with his own protected blocks, making it impossible for you to get to your items until an administrator investigates and reverses the damage. LWC has an option to limit the number of protected blocks per player, but it only takes 6 protected blocks to grief a standard chest, or 10 for a double chest. You can see where setting the limit low enough to block this grief would leave lots of legitimate players with unprotected containers, thus leaving them vulnerable to grief! GriefPrevention's careful tuning around claim size and claim limits makes using your claim to grief someone else's impossible in most cases, and extremely expensive in others (due to the play time required amass so many claim blocks).
Lockette's usage may be simple, but it's not automatic. GriefPrevention protects even the complete noob by creating a claim automatically when he places his first chest.