Thus far, folks mentioning their hardware when posting in this thread have all had the older iPhone 4 model; I'm also running on that generation of hardware.
As the batteries in those phones age, chemical changes mean that they will hold less charge than they used to. I'm observing this in my own iPhone 4, purchased when the model first launched. As an example, if I'm going to get driving directions from the phone at this point, I pretty much have to do so while it's plugged into the charger in my car, which wasn't previously the case.
Regardless of the use of Location Reminders in OmniFocus, my battery is more run down at the end of the day than it used to be. I suspect battery age is a factor in what folks are reporting here, and I'm sympathetic to the folks that want to use location reminders but can't tolerate the power drain.
However, we've verified that the feature is using low-power APIs provided, designed, and coded by Apple. The feature can be enabled or disabled, but we have no further control over the amount of power the API uses.