I can only think of one genuine reason
not to wear makeup: laziness. All the other reasons I have ever heard carry the same weight with me as reasons not to wear deodorant do. They tend to be facetious
and superior, feeble excuses attempting to disguise the fact that the speaker is either too lazy or too timid
to take up an extra daily self-care routine.
To me, putting my makeup on is no different than showering and washing my hair: a part of my daily grooming. Yes, I do it because I'm vain, but then again I equally don't wear laddered tights or scuffed shoes because I'm vain: there's no actual law against looking like a dog's dinner. Why makeup is accorded its own category of vanity is quite beyond me. You iron your clothes, you brush your hair, you paint your face.
First off, I see no reason whatsoever not to try and look my best in all times. That's like saying you don't actually want to reach you maximum amount of income for some silly reason. I've got looks, why on earth not make the most of them? What conceivable argument can ever be made in favour of selling yourself short? I just do not get it - and my firm belief is that in fact, there's nothing to get. It's all the protestations of people who are not prepared to put in the same amount of effort as me, and then try to shift the burden of inadequacy onto me.
Secondly, I consider good grooming to be not only an obvious mark of self respect, but also a mark of respect for others. People at work have to look at me and smell me all day. The least I can do is make that experience marginally pleasurable for them. Fair enough, not everyone shares my taste in clothes or appreciates my makeup, but at least I'm trying to meet them half way. Besides, I find that people, mainly the ones who protest against makeup most loudly, actually don't even notice that it's there. Nobody's ever said to me that I'm wearing too much mascara; but if for some reason I show up to work without it, worried enquiries pour in about why I'm looking so "tired" and "pale".
So it's an expense. Big deal. So is a decent steak or a good bottle of wine. What have you got money for, to sit on it? Your own wellbeing is the most important investment you can make, it's not "wasting it". Nobody's making you buy the most expensive stuff, same as nobody's making you buy makeup that's been tested on animals; I sure as hell don't. Makeup is like food or clothing or any other personal or domestic expense: you do it within your means and to your own standards.
As for people spoiling their looks by applying makeup badly, this is certainly something that happens. However, if there was less sneering about the uselessness and frivolity of makeup, then perhaps more people - women and men both - would find it easier to take an interest, as well as to find information about the best uses of it. I learned everything I know from women's magazines - in most other ways a thing of evil, but occasionally full of useful tips - but a makeup lesson or a free session with a sales beautician in a department store are just as good for getting yourself started on both the techniques and the products you would do best to use.
Don't give me all this natural shit. If you're so natural, stop brushing your teeth and see how your popularity plummets. We're too far away from nature at this stage to use it as an excuse to be plain old ugly.