what the mission of the servers are from the various sites. When you look at what NoScript is blocking when you 1st visit a site, it becomes obvious that some sites just want to get deep in your shorts, and gather/report way too much information, or use bad web coding, or many other forms of bad commerce related to cookies. Many times they aren't malicious per se, but just bad practice. I know I definitely see the performance difference in page loads, when I have 3rd party or maybe even all cookies blocked. However, since many sites require at least a minimum cookie tolerance, one cannot always do that. I also must mention that cookies can prevent repetitive popups or other promotional server behaviors when they are used wisely by web sites to help the server know you have completed an action or seen a page already and gained those experiences before.
It still seems to me that when you get too much of these special text files in there, and you do hit a site that uses bad practices, a good cookie cleaning definitely helps, because at least only the cookies placed by that site are doing any interaction with the server, and no tracking information is being blabbed back and forth from the web site. I set my cleaner to keep cookies from sites I like, and this seems to help establish a little faster browser performance too - occasionally - because the cookie is already set, and the site servers have nothing to do generally but read them, as a minimum, and not set a new one, or add anymore than is necessary, new information in them.
I think most people would do better if they used a passive protection utility that blocked some of these badly practiced schemes in the 1st place, which seems to blind over-reaching communication with said poorly instituted services. It does little good to just avoid the site, because there is usually something useful for the client visiting it. What happens when malicious intent occurs is when something that is actually an active malicious malware gets deposited in the temporary files and goes filching around in account files(including cookie data), up to no good at all - having cookies set by these malicious processes can't be good that is for sure, (for instance - If a webserver sets a cookie with a secure attribute from a non-secure connection, the cookie can still be intercepted when it is sent to the user by man-in-the-middle attacks) So sometimes a cookie is just one more thing you don't want in your computer, and especially if malcode get into the app-data files trying to subvert processes without requesting privileges from the operating system. Most of my clients can't believe how much mischief malware can cause without setting off the UAC, or even a good HIPS utility. As far as I'm concerned I don't want ANY of their files in there, and if my passive protection can block cookies, so be it - fortunately some anti-malware are very good at blocking the IP address of known bad servers anyway, so this can also be prevented.
It has never surprised me how much better the performance of a client's web browsing can become after installing a good firewall, passive protection utilities, and host files to block most of these bandwidth hogging servers, and yes, I don't want their damn cookies in there either. Especially NOT zombie or persistent cookies(LSOs). There is definitely more information about you in some of the cookies, that you may only want a single web site to know, but under malware direction, outside minion servers may receive all the information in the cookie folder, and sensitive meta data too! I mean - let's face it - when malware can read and find a credit card number or SSN in your hard drive in fifteen seconds, I got a problem not believing they cannot break into the cookie folder and do pretty much what they want to there also.
I also want to acknowledge that even good sites can become infected and serve up all kinds of trash; so I'm not always blaming a site that originally had good intent, but for web administration security should still be a priority if they are worth doing business in the future.