It becomes very difficult to get perfect eyebrows (in cases where you use pencils to define your brows) or well defined/straight eyeliner and so on, so it's best to keep them sharp for maximum results but these pencils break/snap in between sharpening them wasting them and leaving them blunt most times.
There are different ways to manage the above prior to use:
Put your lip/eye pencils in side the fridge or freezer before using them.
Leave these pencils to get solid for about an hour (this works perfect to minimize breakage)
Sharpen as usual (you will notice the otherwise soft pencils have hardened enough to be sharpened perfect)
Use your pencils as required, you will notice the will leave application neat and well defined.
Infact these pencils can be kept/stored daily for use in the fridge or freezer for ease of sharpening and eventual use.