Normally the process of evaluation involves a consideration of both pros and cons. There is no particular reason that 'cons' cannot be used without 'pros', but it rarely is. If we are just considering one side, we normally speak of 'points/arguments for/in favour of' or the 'points/arguments against'.
ps. I have just had a quick glance through the first 200 citations of 'cons' in COCA. In those in which 'cons' had the meaning of 'disadvantages', drawbacks', 'points against', all had a nearby 'pros'.
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