It depends on the nature of your complaint.
I work in the complaints department of a UK bank.
Serious complaints that are factual - mistakes on their account, an error made by a member of staff etc - get treated seriously, rectified and potentially compensated with a credit to the account or a gift hamper.
BUT I'd say about half of the complaints we get aren't actionable.
My loan interest rate is too high. I'm unhappy with my ATM withdrawal limit of £500 p day. I'm unhappy I was charged £20 for going £100 overdrawn. Not happy that i have to provide ID to open a new account, although i already bank with you. All policy - tough titties basically. Of course, to maintain the company image they get a beautifully worded letter back applogising that they're unhappy and explaining our policy reasoning and 'I do hope you can understand. We very much value your business'. But essentially, yes, these complaints are pointless. A waste of time.
Then we get service complaints that are unprovable. X in the local unit looked at me funny and had a rude tone. I spoke to X at the branch who promised me this but it never happened. Some people write pages and pages of their opinion on our service, all general and going back months or years - nothing that can be investigated. These letters get skim read and another grovelling apology. Sometimes a token compo of £20 to make them go away.
'We appreciate your feedback on this matter' - if you ever get this line, roughly translated it means 'There is nothing that can be done and your pointless moaning has wasted your time and ours. We have scanned your complaint letter/comments to our system to that we can tick the box of meeting our legal and regulatory obligations of recording all complaints, but it will never be looked at again'.