Yeah, it'* not necessary for the sending unit to be replaced, but it'* like having no fire alarms in your house. Sure, as long as there is no fire, you don't need the alarm. But what about when the christmas tree lights up at 3 AM?
I've ran my truck with a bad sender befroe, and nothing bad happened. The only ill effects are the lack of knowing where your oil pressure is at, and that can be bad. Get it changed sometime soon.
There is a potential problem that could develop if it is not replaced. If it fails all together, it will cut power to the fuel pump leaving you stranded. It'* an engine safety feature so it doesn't get destroyed by running without oil. Murphy'* Law would likely come into play.