While I agree with Skunkape that a cleanse may not work in an acute situation, to imply that we don't need to detoxify the body isn't helpful.
Our bodies have several systems that naturally and continuously remove toxins. However our modern life seems to overwhelm the body's natural abilities. To detox or cleanse simply means that we're removing any unhelpful chemistry from the blood and lymph, perhaps even some of the fat-soluble toxins that get stored in our fat cells.
If someone has gallbladder problems, it implies that the liver is over-stressed and a detox may be in order, to supplement whatever an MD may recommend to correct the immediate problem. It may be even more helpful to change the life-style so as to reduce the load on the liver, kidneys, digestive system, etc. We should never have to do a "cleanse" but many of us have taken in a lot of unhelpful chemistry.
That we don't know which unhelpful chemicals we're removing is not really the point. When we shower, we don't know the makeup of the dirt that is washed away. Are we assuming that there are no unhelpful chemicals in the body? Even though I personally have a lot of faith in the intelligence of the body to distinguish between helpful and unhelpful, I think that there are chemicals in our air, water and food that our bodies were not designed to deal with at the levels that we are ingesting them.