I want my stuff to last over time. Pro will last a lot longer than SR level gear if you treat it right, plus the protection benefits outweigh Senior level protection all day long. It also depends on what piece of senior vs Pro gear we are talking about, brands, etc.
In Beer Leagues, depending on the skill level, guys can REALLY shoot the puck. I personally need the protection and longevity that Pro gear offers, which is why I primarily wear full Pro gear (CCM, in my case).
It also coms down to personal budgets also. I don't want another goalie to break the bank with Pro level gear, but if you can, it really is a good investment, assuming you treat year gear well and are going to be hanging onto it for a good number of years.