I agree with the above for when everything is as it should be..
But in circumstances where a flush would be advisable,
say after a cylinder head has been on its way out, & the head/gasket's been done,
(possibility of moisture in the block/oil )
or you buy a vehicle that someone has been running on veg oil,
(possible there is 'gumming', but a flush will not be certain of getting rid of this)
a flush is worth the money.
(its not very much money really)
Personally as a quick fix,
i just use/buy a couple of gallons on the cheapest oil that i usually have around,
drain the stuff thats in,
fill up with the cheap stuff,
run the vehicle up to temp & drive around and about for a few miles,
then drop the new oil
& change the oil filter.
Then fill up with the more expensive oil, if thats whats going in.
george