here is what I found out elsewhere. I put it as an answer here but I guess there still might be more opinions on this issue around.

The main difference is how your OpenERP installation is being updated. If you use nightly builds and apt-get install (or any other package management system from any other then deb/ubuntu linux) your installation will be updated with every update of your operating system (which you do on a regular basis). There are scenarios you might not want that to be happening. For example you have made changes to the code installed on your system ... an update via package managment will overwrite those and what you have changed will be lost. I think that is one of the reason many people suggest to wirte a new Module for adding a buttopn or field to any form (which you can do with writing a Module though the interface in the 'developer mode').

Installing OpenERP/Odoo using version controll system like bzr/launchpad or now github results into a different way about updates. The Version control system will determine which code is newer and decide based on that whether to update is or not.

Let's say you have added a field 'shoesize' in the personal information of employees using the interface. That is a change in the local code of your installation (and newer then the original code). Now if you update your installtion with github it will detect that the local code for your 'shoesize' button is newer then the one coming from the sources and leave that as it is. All other updates (buf fixes for example) will be processed.

I am not real expert and this is what I understand so far. Would be great to get comments if there is anything wron with this from people who might know it better.