You are absolutely right.
I will never buy a pre-built PC.
There is another thing which some may not realise,
. Branded PCs usually only have a replacement warranty of one year. However, individual components, like hard disks, optical drives have three year warranties. Some RAM chips have a lifetime warranty, or at least a five year one. Not to mention the price would be much lower than the Branded PC sold by say Dell - of the same specs.
So if you buy your own software and assemble your PC, you probably still end up saving a lot, given that the OEM software bundle usually costs a few dollars lesser than the separate one.
Some OS do not allow you to install other OS. or, the warranty may expire. In the home built PC, you can install any type of software you want. I myself dual boot Windoz with Ubuntu Linux, and quite happy.
Laptops, however MAY be better purchased from the vendor directly. Sadly, there are very few laptops that come without an OS pre-installed.
Well, here is to Open-source too !!