In my current project, the team uses BIN deployment instead of the standard GAC deployment. The application is just a custom application using NHibernate as the ORM framework. I know thats not a good practice. BU I would like to hear your expert opinion on why it is not a good idea to do BIN deployment> Thank you for your valuable time.
Paul Beck has a good blog post about this: http://blog.sharepointsite.co.uk/2010/07/deploying-to-gac-vs-bin-folder-in-sp.html
My general rule is: Deploy my custom code to the GAC except if it's not trusted i.e. 3rd party code or there is a business reason/policy not to. It makes dev easier but is not ideal in that best practice decitates that you should apply the minimal levels of security permissions to your code.