Dumb Question For The Experts.

I am no expert, still I'd say that:
- cleaning in most instances leaves hairlines, or small scratches on the coin's surface.
- cleaning leaves some coins dull (aluminum) or unnaturally shiny (silver). It doesn't appeal to me.
For me, it's enough to disqualify a cleaned coin from my collection. Sometimes uncleaned ones are hard to come by, then I choose the one that's possibly least (or long ago) cleaned.

To add to jbuck's comments which were right on, If it happens to be coins in grades of XF40 and above, it tends to destroy the microscopic luster flow lines found on coins with mint luster evident or still remaining. Like jbuck stated, it tends to make an artificial surface which generally throughout the coin collecting community, makes it less desirable and less valuable. It becomes a "problem" coin at that point.

Cleaning has not been taboo until recently. A nice clean shiny coin was preffered over grimy, dirty coins... And if they were not cleaned physically, a lot of coins were dipped to make them appear new. That is why a lot of coins, even those in top-tier TPG holders, have been cleaned or dipped at some point. The difference is the ones that get slabbed retained their eye appeal and have no major damage on the surface from this cleaning.

And that's part of the reason why cleaning is so frowned upon today - because we can now see what damage such "treatment" has done to the coin.

The main problem with cleaning is, if you do a shoddy job, or later change your mind and decide it looked better before it was cleaned, it's too late - a coin can't be de-cleaned. Any attempt at doing so (for example, by artificial toning, or keeping the coin in your pocket for a few months) will simply do even more damage.

Ever watch the Antique Road Show on TV? They almost always stress if you had not cleaned this item it would have been worth XXXXX. There are extenuating reasons for not cleaning items of any kind. The primary reason is some of the STUFF found on coins or other older items if from an actual chemical reation from outside materials onto the original material. With coins it appears as Toning, Tarnishing,
Corroding, etc. On a coin that has been exposed to such substances, some of the original coins material has chemically combined with other substances. For example a Copper coin that has been exposed to Sulfates and/or Sulfites, a greenish substance appears on the coins. This is a Cupric Sulfate and the removal will take away some of the Copper of the original coin. Almost all coins will eventually combine with Oxygen if allowed and this too will form a new substance on the coins and then too, cleaning it off will remove some of the coin.
With Antique Funrniture for example people just have to make it look like new so they sand it down and recoate it with modern varnishes and/or other substances. However, in the sanding process, they have removed some of the originallity of the item.
Many coins that have been found in dirt are just that, dirty. Some very mild and non coin damaging substances, such as Distilled Water of Acetone could possibly remove some or all of the dirt. And without damaging the coins.
Still any cleaning is risky and even dangerous if using chemicals that could explode, start a fire or blind or kill someone. And remember that such an explosion could damage the coins.