Some interesting points.

For customisability, I don't think that this is really a big deal amongst general users.  I've actually never customised a machine further than building it.  And if I did, it'd probably be the RAM/HDD, which you can do on notebooks.  In many cases, upgrading the CPU will require you to change the mobo as well (and potentially many other things) unless you happened to buy a really low end CPU, then plan to upgrade to a high end later on.  Graphics cards aren't really catered on notebooks anyway.  I believe optical drives can be replaced and I can't think of a whole lot of PCI/PCIe cards that an average user would add on (and most of these you can get through USB or the laptop expansion thingy).
Parts breaking can be an issue, though it's something you hope doesn't happen...

Screen: for a normal desktop, you have to buy a screen anyway.  So you can still do it and plug the laptop into the bigger screen.
Your average laptop today can already do many multimedia functions quite well, including watching movies, video encoding and basic 3D rendering.  You'd really have to be an enthusiast to require more.

Theft and permanency...  well, if you don't require portability, still having it can be a bonus if it is portable.  If you leave it at home, you probably have a number of other more valuable things at home which can be 'borrowed'...