I think you've made some valid points in your post. The key on your post however is that you state that you're running small sites, dev sites and email. I encourage everyone (who has the knowledge/skill) to run their own server locally if they're testing things. This will save you money in the long run if your development is going to take a long period of time. The only issue that you might run into is that when you choose a host to put your site live with, you may run into some conflicts with how your personal server was configured and how the datacenter server was configured, but if you know what you're doing, you can get around that.
Personally, my websites have to be online 100% of the time at all hours of the day. Multiple servers takes care of that, and if my ISP would let me (which it doesn't), I'd run a server from home as a backup mail server or for overflow traffic on some small scale sites.
The big issue has to do with reliability and redundancy as Steve mentioned. I maintain just over a dozen servers at local offices for real estate and mortgage and legal firms. Most of these require the servers for internal use rather than external use, but there are 3 small businesses that I cater to that run external facing websites on their server. Works like a charm - but then they have a business line into their office and are setup to operate a server from their facility.
The point I'm making there is not that you can't do it from home, but if you're running a business that requires you to be online all the time (ecommerce etc), paying the $50/month is a drop in the bucket for what it would cost for *MY* services to maintain your office computer! Much like someone who is paying $2.00/month for hosting services can usually afford to be offline for a few minutes or even an hour. If they could not afford the downtime and are loosing hundreds or thousands of dollars per hour (as so many thread state), then really the question is why was the person on a $2/month account
One other item that would go along with your point on #11 is that if you configure your site to also use a CDN you can save a TON of bandwidth and improve speed to having these graphics and common files on an international Delivery Network rather than a single machine.
If you're ever in the market to pick up some older P4 rack mount systems, let me know and I'll gladly sell you some dirt cheap - of course shipping to England might cost you an arm and a leg