Hearing aids these days are much more than just amplifiers. They are tuned to the precise frequencies of your impairment; different for each ear, and may also include programs for Bluetooth TV reception, forward or omni-directional reception, etc., etc. Those settings can be changed by the audiologist, with his computer, and with changes in your hearing over time.
On changing batteries at inconvenient times: You only know when it is time to change batteries when the hearing aid gives you a certain tone signal. It continues to signal every couple of minutes until you replace the battery. The person is not being rude.
Even these modern aids are not perfect. I find them good for certain TV watching; female voices, whispering, background noise, etc.; but, I do not like them for normal conversation with another person who speaks in normal volume. In a restaurant or cine, they would drive me nuts.