The main issue is that you could _not_ boot from it wherever, unless you have several machines with the same model of motherboard. The installation process loads drivers specific to your motherboard and specific components thereon, and a drive moved to another type of machine is likely to fail to boot.
That said: If you get an external SSD, be sure that the case uses the eSATA or USB 3.0 interfaces, or it will be slower than an internal hard drive and you will have wasted a lot of money.
The nearly universal practice among enthusiasts is to use an SSD internally, attached to an SATA III port on the motherboard, and use it as the boot / system drive. The reason is that SSDs cost a great deal of money per gigabyte, and the...