You should always use primer which helps the foundation to stay on top of the primer, use the right kind of foundation for your skin type and always exfoliate, cleanse and tone which always help to remove any debris from your skin. Keeping it nice and clean.
I've been blessed with decent skin and have been using foundation but no impact just as long as I take good care of my skin. Hope this help but at the end of the day, it's your choice what you want to do it's your skin!
In times past, foundation was much heavier than today and it was made with a lot of oils that clogged pores and caused pimples. This isn't true today, modern foundations come in oil-free formulations that don't clog pores as much, and in fact, some come with skin treatments added so they actually treat acne, redness, aging, etc. Some also contain SPF to help protect skin from sun damage. So no, foundation itself isn't bad for your skin.
But if you have persistent acne that isn't clearing even though you are on a treatment program, it is often recommended that you stop wearing makeup to see if this helps. Also, if you are not really careful to get ALL of your makeup off each night, this is bad as it will cause pore clogging and nasty breakouts.
Like prettyinpa said, some foundations that contain a lot of oils and other pore-clogging ingredients can be bad for your skin.
However, I think that a lot of foundation formulas nowadays are actually good for your skin. For instance, there are formulas now that are designed for acne-prone skin that are medicated to help fight acne. Another example is that there are other formulas that calm, soothe and neutralize redness in skins with rosacea.
It is important to read all of the information about a foundation before purchasing it. There are formulas that are non-comedogenic or non-acnegenic, some contain antioxidants and vitamins which are good for your skin and some contain an SPF sunscreen to help protect your skin from UV rays. That's why it's really important to make sure that you are getting a good foundation that is designed for your skin type and that has all the qualities you want.
An example of a foundation that has "good for your skin" ingredients is Smashbox HALO which you can read about here: http://www.sephora.com/browse/product.jhtml?id=P22
I hope this answers your question.
of course it's bad for your skin! you werent born with foundation on your face so without a doubt it is and will always be a foreign substance to your skin. we all know this, however, foundation and cover up has come such a long way. my strong advice would be to start off little and work your way up in coverage. i constantly see women wearing a full coverage foundation ( women who have close to perfect skin that is) b/c they think that that's the way to go when you want a flawless finish. have a color specialist diagnose exactly what he/she would cover up and what products they would use if they were to do makeup on you and go from there. dont forget to read the ingredients!
Foundation can be bad for your skin - even today. It's important to apply foundation and makeup onto clean and conditioned/moisturized skin, using clean hands or tools. If not you can trap dirt and cause irritation/breakouts. Also, some foundations when used on the wrong skin type can contribute to dryness or add to oily-ness. Foundation can be good to your skin if it contains nutrients and protective ingredients, then it can supplement your skin regime/moisturizer. I like the NARS foundations.
Revlon changed some of the ingredients in their 24hour ColorStay Combination/Oily Skin 400 Carmel Foundation. Has this changed what the product does for your skin? After using and depending on this product for almost 7 to 8 years, I found out that the new ingredients do not agree with my skin. I had to go to a dermatologist to see how to treat my chemical burned skin. It was so horrible; I had black and red burn spots on my face. I was so embarrassed and upset!!!!!