In an average setup, a player will have at least one hardener affecting each resistance (usually by way of using an adaptive nano membrane), and usually two affecting important resistances such as armour explosive resistance. One extra module relegates either that module or the rig (whichever is lowest) to the second highest module, giving it a stacking penalty of approximately 87%. Assuming it's the rig, it drops from 30% to a slightly less effective 26.1%, not a huge difference but one that will have a noticeable effect. When a third hardener is introduced, it's goes down to 57% effectiveness, for a measly 17.1% resistance. At this point, it's clearly not worth fitting over other, more useful, rigs.

ARE RESISTANCE RIGS WORTH IT?
For both armour and shield rigging, when considering if the 30% resistance rigs are in themselves effective, even if applied as the sole resistance modifying item on that particular resistance, we must check whether other rigs would do a better job. Since you'll almost certainly have one or more hardeners in effect already if you are tanking only two damage types (for example, kinetic and thermal when tanking serpentis), this makes adding a rig to cover those resistances impractical in most cases.

So let's consider tanking against all four resistances, such as for a PvP fitting. In this case, 30% to one resistance (without a stacking penalty) will always increase DPS tanked to that resistance by 42.9%. If you are hit with equal amounts of all four damage types, this is a 10.7% increase in DPS tanked overall. By way of comparison, the 15% repair amount rig increases the amount of DPS tanked by 15% overall, putting the final nail in the resistance rig's coffin.

This also applies to the shield resistance upgrade rigs, though the argument is slightly different. Since there is no shield boost amount rig, instead we use a capacitor recharge rig. Providing that we aren't already able to run the booster constantly, it will increase the number of boosts we can make per minute, increasing damage per second tanked by over 10.7%.

ARMOUR AND SHIELD
There are some differences in the remaining shield and armour rigs. Both shield and armour have repairer/booster cycle time rigs, which increase their cycle time by 15%, but only armour has the repair amount rig. Shield obviously has a shield recharge rate rig while armour, of course, doesn't have a recharge rate to begin with. Both have rigs to increase hitpoints, a 15% bonus for 50 calibration.