Why "It's just Common sense!" arguments are weak.

Lazy leaders attempt to manipulate some one's intelligence ego when they use the "It's just common sense!" argument.  Either way, if you agree or not, with their cause with only that reason, places you in an awkward position. Accepting the cause puts you in an accepted role in a perceivable intelligent group,while disinterest, could get your own perception of common sense challenged.  

Manipulation in the making.
The common sense reasoning is a "manipulate the masses" tactic.   A leader will attempt to use the common good for the whole majority -group think- strategy. A strength of united fronts plus, acceptance in a group, fuels the goal further. Not only are they using the drive to work together belief, but also the peer pressure factor.

If a person doesn't agree with the 'it's just common sense' argument, their intelligence can get questioned, as the arguer tries to instill emotional intimidation. A reach to insult self confidence. If a person accepts the argument with only that remark, they're falling into an easy quick, follow the crowd, decision.  

Common sense isn't common
Upbringings vary so diversely that the rate of total acceptance on one particular reason, across vast different groups, is rather low. Morals, ethics, and common sense are all individual interpretative values based on what was exposed to in the direct environment.  For example, one who was surrounded by, or close relation to,: hunters, military personnel, or police force may support less gun control, versus one who was brought up as a peace loving charitable humanitarian. Both would carry differing opinions about the use of guns. 

If you don't agree
When you don't agree, your lack to follow, can get your own common sense decisions judged. Personally, I feel if a person argues with only that lame debate retort of "Why wouldn't you? It's just common sense," rather than providing enough supportive convincing specific debatable facts, it actually, makes them appear less knowledgeable about the topic. If they are not willing to give their reasons, other than that excuse, they are not as convincing to sway others and you should, in turn, question their own intelligence on the matter. 

If you agree 
If someone agrees quickly with the common sense argument, rather than inquiring further data, they are lazily accepting the cause, partly from a sense of wanting to belong or be accepted. If you agree with the cause with only that excuse, your sense of ego is strengthened by the emotional feeling of appearing smart by smart peers.  

Be an independent critical thinker
A true intelligent decision comes from research, experiences, and feelings. Don't fall prey to the lazy leader who only appeals to your feelings rather than provide several other factual reasons.  Common sense is not a strong enough argument and should be a trigger warning sign to be cautious of personal intent. Major decisions should be given more serious thought, especially those causes that will truly effect the masses.