Besides that costs incurred using either CO2 or N2 as a possible fire suppressant, you also run into the physical dangers using gasses in such a manner.
Nitrogen in amounts higher than atmospheric normal (78%) can cause mental confusion, unconsciousness and death as it displaces Oxygen in the bloodstream. We used N2 to purge electronic assemblies to prevent condensation and our safety procedures required high flow ventilation even though very little N2 was released into the atmosphere of the shop. An "uncontrolled" release such as you suggested would require either a ventilation system capable of completely replacing the entire volume of air in a shop within a minute or evacuation of the area until safe. Even when purging in an aircraft hanger we were required to use large, high-capacity fans to prevent high concentrations in the area.
Since we repaired electronic assemblies as well as mechanical, we used three types of fire extinguishers in our shop: dry chemical, CO2 and Halon. ANY discharge of CO2 or Halon REQUIRED immediate evacuation of the shop until cleared by the fire department. A dry chemical discharge only required using a face mask until it was cleaned up.
Any gas continuously released into an enclosed area, even well ventilated, can cause adverse effects to the human body in seemingly insignificant concentrations. CO2, N2 and Halon will quickly asphyxiate somebody by simply preventing Oxygen from entering the bloodstream. In a small enclosed room as described by Lord Redacted, any continuous release, however small, would quickly raise gas levels to dangerous concentrations. Add to that the costs of using compressed gases (tanks, transportation, hookup, etc.) could quickly result in uneconomic final pricing. And an inspection (safety, business and insurance) would fail unless an aforementioned atmospheric or enclosed worksite evacuation system was installed at high costs. Either way, while seemingly simple and easy, the uncontrolled usage of gases for possible fire suppression is too dangerous and costly to be considered for a small business.
NOTE: As a volunteer firefighter, entering this workshop while the gas is being released would be considered as a "hazardous atmosphere enclosed area situation" that would require the use of SCBA (Self-Contained Breathing Apparatus) and an atmospheric monitor. SCBA wouldn't be removed until gas levels returned to normal and levels continuously monitored until the call was completed.