Now, I have to repeat myself:
Internal only during backup, then take out again and put aside.
I hope, I made myself clear now!
Internal is cheaper and faster, so why (apart from plugging in and pulling out) should you consider anything else?
All right, all right, I missed the part where you said to remove it afterwards...
I'm still not convinced of this method of plugging in/out internal hard drives though... Granted, they're cheaper than externals and SATA interface is certainly faster than USB, but even as you say, there's the issue of convenience, opening up you PC box and plugging in a hard drive is, at best, incovenient for someone used to it, and a plain unspeakable nightmare to someone who isn't. Whereas external drives are operated as easily as pen-drives or memory cards (provided you own a reader), you just plug them and you're ready to go, I don' t know... most friends of mine would be all but scared to death of having to open their PCs and mess with it in order to make a back-up. Let alone if at some time you decide you'd like to use that hard drive to interchange data with an external system (say, a friend's PC), something quite common nowadays.
To sum it up, I think the internal HD method is best suitable for those who have some confidence at handling the internals of a PC (and expertise, as Gordon says, it's all to easy to brake things in there, that is not so with external interfaces, designed to be used more often and by user-level people). My humble point of view, of course...
Anyway, and I forgot to mention this is my previous message, it's a great idea to have a back-up drive, either internal or external, but my advise is that, at least the most precious data, should also be backed up in permanent optical media (such as DVD-R) and have that media archived in a proper environment (basically a dark, clean and dry place).