a useless vs an useless

Some examples from the web:

• While people may be familiar with these points, it is not a useless exercise, in my opinion, to reiterate them today.
• Mr President, Commissioner Byrne, the incredibly detailed draft regulation that we are debating this morning is a perfect example of a useless text.
• It's a useless waste of time.
• A princess is such a useless thing.
• I'm a useless old man.
• So why bury a useless knife?
• You know what a useless minefield it is.
• Han Se Gyeong was a useless person.
• You know, maybe sell off a useless airfield.
• Better to me than a useless drunk.
• It is a useless, crippling emotion.
• We all know what a useless man your father is.
• Now Santa can bring you a useless collection of ballpoint pens...
• That's all, nothing more than a useless exercise in deduction.
• My pop-pop once told me that guilt is a useless emotion.
• It's just a useless hunk of metal.
• What? Guilt... is a useless emotion.
• I am not a useless mortal like you are.
• The whole idea of marriage is a useless vestige of an archaic civilization.
• It's like a useless Good Will Hunting.

Some examples from the web:

• Yet you're going to abandon it and become an useless and insignificant human being.
• We're travelling without a map or with an useless map.
• Isn't it enough you built those slums in an useless bog that's good for nothing?
• I would if you'd stay. but since you're going. it'd be an useless effort.
• It is a piece of sheep, an useless heap of flesh to all.
• Parliament is content with an innocuous and useless resolution.
• What an extraordinarily useless and twisted branch of the tree of knowledge.
• Not we will have an ideological useless debate.
• This process is very efficient and produces energy from an otherwise useless by-product.
• Conventional weapons are useless against an Angel.
• It's an absurd and useless hypothesis.
• Besides, an electric car is useless at the moment.
• Without someone on the inside to lower their defenses, turn off the acid fog, an army is useless.
• Without someone on the inside to lower their defenses, an army's useless.
• They're normally useless on an ordinary photo, but I thought it just might...
• Meaning, I'd be useless on an expedition.
• Only in that way can a European force of this kind put an end to the useless duplication of defence efforts among Member States.
• Baroness Ashton, we can all remember the Copenhagen climate change summit as an example of the useless cacophony that is brought about by over-representation of the European Union and its Member States.
• ...as well as an obligingly blind and useless husband, meaning that...
• The tree canopy makes satellite imaging useless.