The best reasons are that hose rated for propane service is suitable for propane. Is hose rated for CO2 suitable for propane? Perhaps it is and perhaps it isn't. Does propane degrade the plastic in the tubing over time? I don't know and if you don't either then I would stick with the material that is certified for the job. One aspect of this that people often don't think of is that insurance companies are awfully clever at dodging payment of claims. If there is a kitchen fire from an electrical fault and they find a non compliant gas device in your garage they may be able to say that you violated the fine print in your policy and are, therefore, not covered.