I do not think those disadvantages apply in most cases. Back when DSL was new I used an old 486 for many years, this is not new territory for me.
Size is irrelevant unless you're putting it in your entertainment center (why?) or live on a sailboat. Just stick it in the basement, stack on top of the fileserver, whatever. If you're doing the dorm room thing or living aboard a sailboat you have to realize that involves some highly unusual lifestyle compromises. For most people its not an issue.
Noise: I did splurge on some large slow fans (like $10) to replace old small fast (LOUD) fans. Again this is a lifestyle thing, where if your castle has no location further than 5 feet from your sleeping head, you're going to have serious lifestyle issues that most people simply will not have. I don't find my desktop at home or work to be particularly loud. I did at one time run a more modern desktop as a firewall and specifically ripped out the fancy graphic card and used the on board video, to reduce noise a little. After installation I never used the video or keyboard again, all SSH access, so its not like it needed fancy graphics. My main firewall/wifi/dhcp/asterisk/stuff/etc box is about 50 walking feet from my sleeping head, past the (sometimes) loud fridge, the dishwasher, the clothes dryer, the hot water heater... For most people its not going to be an issue.
Power consumption. Not an issue. I was drawing about 50 watts which will cost about $50 or about 3 weeks of cablemodem service per year. Using the EE tradition of it costs about $1 to provide 1 watt for one year. I admit I was an idiot and upgraded to a soekris box many years ago which is basically a 5 watt PC. So I save about $45 per year of damage to my finances and the environment. Great, that'll only take like ten years to pay off the capital expense / manufacturing environmental degradation. That was a dumb move on my part and I'd suggest you're always better off both financially and environmentally by reusing an old desktop.
Lack of ethernet ports (LOL, serious? Plug in another board?)
>Size is irrelevant unless you're putting it in your entertainment center (why?) or live on a sailboat.
No, it's not. If you're using *DSL (including FTTC) then how much space you have available depends on where your phone line enters your house, for instance.
My lines comes in, from a pole in the alley behind my house, to my kitchen and the master socket is by my kitchen door in the hall. I certainly don't have room for a large box there, but a typical router fits nicely.
If you wanted to sensibly advise using pfsense you could point at the zrouter project http://zrouter.org/projects/zrouter/wiki/Supported_devices