Lack of definition and verifiability
The description mentions two criterias for this tag: the lack of vegetation and the aridity (lack of water). Both of them are not verifiable and this makes use of this tag rather counterproductive:
- the lack of vegetation in deserts in not absolute, on the contrary many of the areas currently using this tag contain significant vegetation. But there is no definition how much vegetation can exist in a desert.
- there are already verifiable and well established tags for areas lacking vegetation, namely natural=sand, natural=scree and natural=bare_rock.
- in many area with sparse vegetation visibility of the vegetation varies strongly with season which makes verification be the mapper difficult even if there was a clear definition of the amount of vegetation in deserts.
- aridity by definition is something that can only be verified by long term statistical observation and even then only with very limited spatial resolution.
- there is no clear established definition of what a desert is even if climate conditions are precisely known - see 
This lack of definition is also visible in the data - most of the desert areas have very crude outlines due to the lack of basis for a clear edge position. Where there are articulated outlines these rarely define the edge of the desert as a natural form but are edges of settlements, edges of sand covered areas as opposed to rock deserts etc.