Quoted from SkitBPinball:
Shaker Motor: Only if the theme really called for it. The only example I have is Jurassic Park, as that was just perfect. If we ever did anything that would really benefit a little more than a "look how many features my game has" sort of way, I wouldn't hesitate to add it in.
Shaker motors are more important for some themes than others; I totally agree. That said, shakers are one of the few non-optic add ons you can do for a pinball machine (literally the only other non0optic add on you can do is an improved sound system). In my mind, ANYTHING that gives extra feedback to a player is great, few things give as much feedback as a shaker.
While I agree that you guys shouldn't incur the cost of installing them, having the game wired and drilled for them is super handy as most people who buy NIB games buy shakers too . . . at least that is my observation.