This was something I considered very recently (when I realized my only remaining VGA monitor didn't support the oddball frequency put out by an old Supermicro server on which I was still running something important).
Some pros of laptop servers:
* Quiet.
* Low-ish power consumption.
* Doesn't take up much space.
* Built-in secure console.
* Built-in UPS.
Some cons:
* Limited in what drives you can put in it. Some laptops only support one drive, so you can't even do RAID mirroring. A real server will usually let you put at least a few high-capacity 3.5" drives in it, on SATA or better.
* Many laptops will overheat if run with the lid closed.
* If you get a burglary, a laptop is very likely to be stolen. (In a rack in a city apartment, I bolted down the servers with security head screws, and there was no way they were taking the whole cabinet, with a huge APC UPS anchoring it down.)
I use a few old laptops like this, but I opted to remove the batteries due to the risk of spicy pillow.
The benefits of the "built in UPS" didn't seem worth it to me because if my power is out, my internet is down anyway, and I don't run anything that needs that much uptime anyway.
Hard shutdowns can cause data corruption. So, it doesn't need to be about server uptime as much as just not having a program's database get destroyed because the power went out for 10 seconds.
Some pros of laptop servers:
* Quiet.
* Low-ish power consumption.
* Doesn't take up much space.
* Built-in secure console.
* Built-in UPS.
Some cons:
* Limited in what drives you can put in it. Some laptops only support one drive, so you can't even do RAID mirroring. A real server will usually let you put at least a few high-capacity 3.5" drives in it, on SATA or better.
* Many laptops will overheat if run with the lid closed.
* If you get a burglary, a laptop is very likely to be stolen. (In a rack in a city apartment, I bolted down the servers with security head screws, and there was no way they were taking the whole cabinet, with a huge APC UPS anchoring it down.)