Not if it's a vanity investment (I don't know if there's a better term for it). If some VCs know that their fund will ride on one or two unicorns (or network effects like in YC's case) and that the rest of the investments will fail or barrly return 1-5x, once it's clear what the unicorn is there's a little room for investments that aren't expected to have a financial ROI. Perhaps the investment gets the investors a foot in the door for future ventures by the team or into a field where they might not have deal flow, or they might just want the extra "street cred" and publicity. YC regularly gives slots with funding to nonprofits now, which by definition can't have a financial ROI.