I've always been the type to purge items in my closet pretty regularly. When I lived in Boston I used to do it when I switched out my Fall/Winter and Spring Summer clothing (was a good excuse to look at some items with fresh eyes after not seeing it all season and decide whether you really wore it enough last winter to merit hanging on to it.
Sometimes I purge things because it turns out I just didn't wear a piece as much as I thought I would or because basics in my closet have become worn and need to be replaced (like t-shirts and such) and more recently I've been purging a bunch of stuff that is more "professional" looking than I need to wear at this stage of my life. I used to speak at a lot of industry conferences on Ecommerce Optimization and User Experience, so I had some nice "professional but still uniquely me" pieces.
Over the last couple of years I haven't been doing speaking engagements so I've had no cause to wear them -- so I've been selling them and replacing them with things I'm more likely to wear regularly (cashmere sweaters, moto jackets, boots, etc.).
I've kept a few pieces, knowing I will end up speaking professionally again but just not as often as I used to (so this way I won't feel the need to go out and buy more "professional attire" when I do have a gig).
I've also purged a huge number of dresses and skirts (unless they work well with tights). Not *all* of them, but definitely more than 50%. When I lived in Vegas and was working at Zappos the weather was more often than not VERY hot (which I loved and found comfortable) so I wore a ton of dresses and skirts. Now that I'm living in SoCal and the average temp is around 70 it's just too cold for me on most days to go bare legged (I am almost always cold and don't start to feel warm till the temp passes 85 or so).
I realized I likely wouldn't wear these things regularly anymore so I kept a few for when I visit Vegas and other hot climes and either donated or sold the rest.
My last reason for purging is problems with my feet. I developed arthritis in my left foot so I can't wear many of the sky high heels I once could. So I've been selling and donating a lot of shoes from my extensive shoe closet (working at Zappos.com and Onlineshoes.com came with a hefty employee discount, so I definitely have too many shoes!)
I still shop often and sell items often. Since I buy most of my stuff secondhand (via online sites like ebay, designer consignment and thrift stores) it doesn't really hit my budget too much at all --and since I regularly sell items I don't wear I find that what I get for those pays for many of the newer items. So it sort of balances out and is a nice hobby to distract me when I need to be distracted from work.
That said, as a former antiques and vintage clothing dealer, I also have pieces I've owned for over 25 years and would never consider purging, no matter how often they are or aren't worn (like one of my vintage Pucci dresses).