The more inexpensive LED indicators only give gross indications but they also act as a secondary reminder that the switch is on. Sometimes this is worth more than the actual charge state.
I've found I left my plane turned on before because I noticed the lights.
That's 10 bucks well spent, to me.
I dont see why people buy into these gadgets and keep spending money for every new plane when they can buy a quality voltage checker once and be done with it.
Because some of us have only a basic understanding of how electronics work, and don't know any better.
I am willing to learn, however.