Actually, it's a pretty common fix when a mod author is including outdated apis. However, mod authors should not be including them, but to each his own.
I am unfortunately forced to include some of them, including BuildCraft, CoFH, Waila, OpenComputers, and Minechem because of classloading errors. Basically, even if I use @Optional or @Strippable to remove implementation of interfaces, some of them still cause NoClassDefFound errors on the missing class. More strangely, for those classes, if DragonAPI is installed, unless it contains the APIs,
even the parent mods cannot find their own APIs in their own jars! Noone thus far has any hint as to why this is, and the only lead I have is that my mods are being loaded with the wrong classloader (LaunchClassLoader instead of the mod classloader), but that is entirely beyond my control.