Quite true, but if the host is at all busy it's going to get nasty.  Highish utilization (as long as you're not running at 90 - maxed out all the time, obviously) is generally the way to go.  Also, with VMware (no idea about HyperV, we don't use it on our SQL Server infrastructure) if you're under provisioned and need more cores, that's online.  If over provisioning is killing your performance, removing cores requires a reboot to take effect.
In terms of what it's sharing with - I really like, and I mean really, really like if your infrastructure can work with it - SQL Server dedicated infrastructure where possible.  SQL Server and other stuff (app / file server) doesn't play nicely in a lot of cases - although with some stuff you're going to find SQL Server and app server on the same VM is your best call.