True but then you are simply using empty capacity of the others which is not guaranteed to you. So if the other customers start pumping your connection speed drops.
Not necessarily if we are on a dedicated Gbit port (which we are at least at Axigy) and the ISP has enough upstream capacity. Limehost now only offers "best effort" shared Gbit. Back when we ordered our server, it clearly said dedicated Gbit.
Also, we don't really care as the deals have already paid out. We only make monthly contracts so we can easily move in case something happens. There's no reason to pay extra just because.
My strategy was to go through web hosting forums and pick out very cheap ISPs. FDCservers for example claims to give away "enterprise 10Gbit, dedicated port" for $599 at the moment. Who cares if it's "just" 2Gbps in the end. It's still a great deal. We have been kicked from FDC in the past and they don't have RIPE IPs so we're not going after that deal - it might still be good for running some fast non-exit relays.