I didn't mention it initially, but the harm it can do is overriding newer versions. The 3.5 and later installers have proper versioning built in, but I don't think that's possible with the 3.4 installer as well (due to MSI limitations, not any deficiency of the previous authoring).

Of course, we're already in a place where "last installed wins", so the only other option is to remove the launcher from 3.4 (or all installers...) and require a separate installation.