Rechargeable Batteries

[Mobius1]

Many electrical products and appliances warn not to use rechargeable batteries. Some warnings are more serious than others. For example "we advise against the use of rechargeable batteries" compared to "should never be used with this product".

Sometimes these batteries are of a marginally lower voltage and usually contain less energy (compared to premium retail batteries) if I'm not mistaken (mAh?) but is there any real danger of such batteries damaging an eletrical product/appliance? I've heard people say that it's just a way for them to encourage disposable batteries to be bought.