Yes, most definitely. I set up gmail to automatically add everyone I reply to to my contact list and in my job today I often get 5-10 e-mails per day from potential customers, investors, partners, etc (sometimes many more). I have to be able to follow up with any one of them at any point, and while I don't need to have them in my contact list (I can search for their e-mail and copy-paste their address), it would make my job much more frustrating and time consuming. I haven't hit the 10k limit yet, but I suspect I will relatively soon. This is actually a very common use case for a huge number of business customers, it's certainly not an isolated edge case.
Sounds like an application for a database or CRM system of some sort so that you can age out the old entries. How many contacts from 3 year ago are still relevant?
True, but a similar argument applies to e-mail. How many e-mails from three years ago are still relevant? The main value proposition of GMail when it came out was that you never have to delete email anymore. Why not do the same with contacts and take the hassle out of it?
Besides, most modern CRM/ERM systems interact with e-mail (where you bcc the CRM system to add data to it). So adding a CRM system only exacerbates the problem.
It depends on what you do. Personally, I'm a pretty meticulous filer, I keep lots of paper and digital records, filed in filesystems so I can figure out wtf happened 3 or 5 years ago. (I'm a manager in a large IT organization)
Ancient email is often important to establish the context surrounding decisions that have multi-year impact.
Also, many folks use email as an ad hoc general filing system. They rely on search to find information, and email is often the ONLY system available with reasonable search ability.
If you're in a small company, this will sound insane. But in a mega-sized org like the one I work in, its essential.
