What BizTalk application, orchestration, schema, map changes are allowed to be not forced to import MSI via admin console but only install DLL in GAC?
Importing via console force to stop orchestrations and terminate instances but installing in GAC only require restart hosts of this application. So it will be sometimes convenient to not to stop everything on production environment.
There are big risks if you just GAC a DLL in Production without importing it into BizTalk.
That if for your next deployment, you generate a backup MSI from Prod, it will contain the old DLL that is in the BizTalk database, and not the GACced version. This could mean that if you had to roll back using that MSI, you would lose your patch. We've experienced this when someone from another company had done a patch, and not only that, had not checked the change into source control, which was why we had to roll back in the first place as that change was not in the release package.
Another scenario that after the deployment has to be rolled back is that you use the previous MSI (pre-patch) used for deployment previously and forget to re-apply the patch. Again this will cause you issues.
If there is an issue and the schemas or maps in BizTalk don't match the GACced version, it makes diagnosing any issues a lot harder.
In summary don't do it, do proper deployment packages instead that are generated from a build server (so only what is in source control end up being deployed).