The hot-standby server can perform polling duties for a single failed poller at a time.
You configure the hot-standby to be either automatic or manual.
In the automatic case on the hotstandby server, you configure the pollers that you wish to perform a hotstandby function for, and the time parm (i.e. how long the poller needs to be down before the hotstandby starts up).
It might be worth asking Sales about the future of the hot-standby server. Now they are moving towards the jobengine type model of polling (and the non APM, IPSLA module support), there must be changes planned. I'm hoping that the hot-standby becomes redundant with some kind of ability to dynamically move nodes between polling engines (at least without stopping services as we do now).
I don't think it's anything to be concerned about however - in my experience SolarWinds are good at ensuring your investment is protected.
As long as the future will allow for a "favored" poller to be defined for nodes (and that poller will again become the primary once it's restored) and perhaps a favored [set of] secondary poller[s] I'm on board.