Default Site Collection or new Site Collection for Intranet?

I've created two Intranet sites using the root site for the tenant (different tenants, obviously), with no problems, one has been running for 3 years and the other for 1.

Vardhaman's first point is to do with site creation against the root site, well, as an admin I'm in charge of that and we don't do it, in fact who would use that facility? We create site collections using a SharePoint add-in that we wrote which gives us more control over the sites created and we can add metadata for the site for auditing, security etc.

His second point, well I have build scripts that can build the Intranet site against any site that I want, including the root site, again no problem here.

Third point, I'm not sure why you'd want to do this really. When developing the Intranet we develop on a dev site i.e. /sites/intranet-dev, UAT is /sites/intranet-uat, we have training on /sites/intranet-training, finally when we go live we deploy to the root site. In my experience with large Intranets, which ours was, we are deploying functionality in an Agile manner so we went live with parts of the Intranet such as News first, then departmental sites/pages then our other content. We were also migrating content from another system so this approach allowed us to be flexible with our publishers so they could be given the time to organise for the migration and to do the migration, which can take some time.