CD lense cleaner. Is it necessary ?

NO NO NO!
The lens assy is a very delicate device.
Cleaning it without need is a waste of time and could leave you with a broken or damaged CD device.
It is true that rarely does anything 'bad' happen from cleaning, but then nothing good comes from it either.
Leave it alone unless it is mistracking or skipping more than normal. Then a cleaning of the general area around the lens may do more to fix any problem than cleaning the lens itself.
Clean. dust free CDs are the best thing for a CD player. I use a (softened tips with sandpaper) cosmetics brush to clean the particles of dust off.

Yes many manufacturers {Marantz for one} are recommending to stay away from those self cleaning discs. My answer is somewhat a combination of the first two posts. If your machine is exposed to dust or cigarette smoking over a long period, Pop the top and clean it yourself. Its very easy to do with most machines. If your CDP is in a non dust attracting smoke free enviorment, and you can honestly say you have never heard any sound degradation, then I would leave it alone.

I can hear the improvement after cleaning even on boomboxes. Dust accumulates in time. What kind of nonsense is this not to clean the lense? True, one has to be careful not to scratch it and to use an appropriate fluid.

I was ready to get a new DVD video player since certain discs were not being read by it while playing on other players, until a friend suggested I try the Radio Shack cleaning disc.
They had several to choose from. One was for DVD and CD palyers one was for just DVD. It was hard to see what the differance was but I purchased the one for DVD.
It worked very well and all the discs that wouldn't play now play perfectly.

If you live in a dusty environment, lens cleaning is mandatory. The dust build-up over time will definitely affect the sound. I have never damaged anything using the cleaning disk. You are more likely to damage something using an anti-resonance mat.