There are several reasons why I would do this:
Better Hygiene - the eye area can have the biggest buildup of bacteria. This is also one of the reasons why you NEVER share mascara with anyone. At the makeup counters, they will make a big show of taking out a new wand to use when they are putting mascara on you for a makeover.The problem with this is, they dip that "new" wand right back into the tube when they are adding more, instead of getting a brand new wand, therefore defeating the purpose of the new wand, and keeping the tube of mascara hygienic.
Another reason is that the mascara dries out after about 3 months, and is no longer doing the job as well as it was meant to do. Pumping the wand does not make things better, it makes things worse. It pushes air into the tube, thereby compromising the mascara's integrity, and it dries out faster.
To keep track of how long you've been using your mascara, get a black marker and write the date on the tube. If the tube is a dark color (like Loreal's black tubes) write the date on a piece of masking tape and mark the tube that way.
Voila! Not only will you be hygienic, you will have plenty of opportunity to play with new mascaras and not feel guilty!
Here's to long and glamorous eyelashes!