A lot of startups get sued by Non Practicing Entities (trolls) that have no operating business to speak of. That provides the slimmest counterattack surface - so not sure how useful it is to have a portfolio to hit back.
I think a better defense mechanism would be a legal defense fund that would force the trolls to make their case at trial, to go through discovery, to bring expert witnesses.
Because the law firms that represent them work on contingency, this would effectively cut into the potential licensing fees, and make trolling less profitable (and less likely).
Also, a lot of trolls extract patent licensing fees out of startups for patents that should have never been granted, and that deserve to be invalidated (e.g with prior art), but the process of getting a patent thrown out is expensive, so having a fund would greatly help.
The other strategy that could work is to get all the startups that get sued by a troll for a specific infringement and make a sort of "reverse class action" - making it possible for the startups to re-use the same lines of argument, evidence, etc in their cases.