Is there any reason not to set up the install so that major upgrade is always done and productcode changed?
I find that supporting different kinds of installs needs more
I generally find that minor upgrades are only useful in a few cases:
1: Uninstall Bug: There is a bug in the uninstall sequence of the installed MSI. Since a minor upgrade does not uninstall the product, but merely reinstalls it. This allows you to fix whatever was wrong in the uninstall sequence before it is called.
2: Hotfix: There is a very small tweak required for a very large product. Say a bug fix for a single file or a few files. There will generally be a small bug requirement after a couple of weeks for all large products where you need to replace just a few files. Major upgrades can be terrible for this - uninstall and reinstall for just 4 out of 10000 files?
3: Patching: You want to deploy a patch for your product. MSI does allow major upgrade patches, but these are even more complex to get working than minor upgrade patches (which are also difficult).
The "Small Update" feature should never be used for any purpose in my opinion. Major or Minor upgrades only.