To be honest, I don't believe that primers are a must. It's true that women for many generations have used makeup without needing them and so why now? While part of me believes that some primers are indeed effective (I'll get to that later on), some are just out for your money. While I never use a face primer, I've tried the UDPP and it works great. However, I don't use it everyday and I recommend a primer only when necessary, such as maybe on your wedding day or when you have to wear a full face of makeup (i.e. concealors, foundation, blush, contour powder, 8 eyeshadow shades, mascara, false lashes, brow powder etc, etc) and make sure everything stays in place for longer.
The truth is, most girls don't head outdoors with that much makeup all the time. Some primers are meant to "protect your skin from the makeup" that you use, but if you use the right type of makeup, you definitely don't need a layer of protection! I mean, if, for example a certain foundation breaks you out, would you use a primer underneath it to prevent breakouts or would you just change the foundation altogether? Right?
Even where eyeshadows are concerned, primers are not a must unless you want really intense colours and get the most out of your shadows, and/or you are heading for a rock concert where you'll be perspiring after a while. As a final note, I feel that primers could be just another thing in the beauty industry that companies come up with to make you think that your skin is not healthy enough (or doesn't hold makeup well enough, or just not good enough for whatever reason) and needs more help.
xoxo,
Roxanne