It's fine until your garage burns down / floods, or someone digs through your street's connection.
Those things are unlikely, but even with a backup you're going to have pretty horrendous amounts of downtime. You could lessen the impact by having some off-site secondary system ready to go at any moment, but then at that point you might as well be renting a server.
More than calamities, I'd worry about simply going away for a week or two and having something happen. With Linode/whoever, even if I'm somewhere remote, I suppose I can at least call or write email and get some help. If everything's at my house, what would I do? Leave the keys to a friend? "Can you feed the cats and take care of the servers in case they have a problem at 3AM?"
Well, according to this article [1] a burglary happens every 15 seconds. I highly doubt any government agency is that efficient, but even if it did happen and you have a backup you could be back live in a couple minutes. Just like the scalability comments below allude to, the key is having the infrastructure right and then everything else is easier.
worst comes to worst, I can walk into frys and throw together something shitty. (note, I don't recommend running prod. on Fry's hardware... just saying, if you said I needed a server in two hours, I could do it.)
Besides fire/flood/theft/etc, is the garage clean and climate controlled? I used to run a server in a basement during college, and burned through 2 disks and one motherboard before I finally figured out that warm, damp, and dusty basements with dirty power supplied by 1920's wiring weren't a great place to run servers.
For me, it's a peace of mind thing. I have better things to worry about than hardware.
Also, read the fine print on that Comcast plan. Their residential service is now capped at 250GB (not sure about business-class). In my experience, Comcast is the last company I would want involved anyways. Our service goes out frequently enough that I now have a backup WiMAX modem for my laptop.
Those things are unlikely, but even with a backup you're going to have pretty horrendous amounts of downtime. You could lessen the impact by having some off-site secondary system ready to go at any moment, but then at that point you might as well be renting a server.