Here's something I've been wondering about. As the corp, when is it worth it to purge viruses to get rid of the virus counters?
Ignore all the complicating factors. Imagine there are no agendas about to be scored, no unusually vulnerable servers, etc. Just other things being equal, when is it worth purging to take away credits?
My first thought is that the cutoff is 3 counters. I'm usually happy to spend 3 clicks as the corp to take $6 from the runner, but I wouldn't spend 3 clicks to take $4 from them. But I suspect that's an overly simplistic analysis... it doesn't sound right that putting a Fermenter on the table takes away every third turn from the corp forevermore.
The problem is, as you say that even if the corp purges, the fermenter is still getting counters.— valerian32
The alternative to purging is rushing. Force the runner to pop their own Fermenter, because you make them need the money :)— HolyMackerel
The runner gains a click when you purge (i.e. they save the click they would've had to spend trashing fermenter) so I think it's better to purge when it gets to 4 counters— Lttlefoot
I usually purge at 5, sometimes (when a scoring window needs the runner broke) 4. The total cost of fermenter is 4 (draw+install+install cost+pop), and your cost to purge is 3, so it doesn't become good until the money they'd get-4 > 3, aka 4 counters, but an entire corp turn is usually way more important than just 3 credits— m.p