Happens constantly in the LTMA. Pilots get airborne and call "London Control, ABC123, SID123A, passing 3000 for 6000, Squawking Ident".
The controller should respond with an instruction to squawk ident... because of the reasons already given.
Invariably, the pilot will not read back the instruction (which is wrong because they are supposed to and more importantly it is also the final step in proving serviceable 2 way communications). Observance of compliance with the instruction means nothing if the pilot dos not respond verbally - although we know that it is more than likely identing in response to our instruction, as Grizzled said, we cannot assume.
Also, although I don't care because I know I am in the right, I often get the feeling that pilots must think that ATCOs do not listen to them, because after all (as far as some pilots must be thinking), they got airborne stating they are squawking ident - if ATCOs were listening properly, then they wouldn't ask pilots to squawk ident again
I
think the reason pilots do it is because they know they will be asked to as soon as they are airborne, therefore they mistakenly believe they are helping us out!