Although I've never had the need to enforce it as a DM, the PHB does have one more nasty effect of the death of a familiar. "A slain or dismissed familiar cannot be replaced for a year and day." If you base your strategy or build around having a familiar and that familiar dies, you're going to have to have it raised from the dead or come up with a new strategy for a year and a day of game time.<br/><br/>Familiars can also be useful out of battle. If I recall correctly, there's a spell for switching places with your familiar. If you have a bird familiar, it's a great way to get to the top of some structure, even if line of sight is required.