Why Royal Fleet Auxiliary ships are not armed merchant cruisers

Can RFA ships be considered able to fulfil armed merchant-cruiser roles?

No. They may be armed, but they are supply ships and should not be treated as combat vessels for those roles.

Royal Fleet Auxiliary vessels are supply ships first, not substitutes for armed merchant cruisers. Their weapons provide limited protection rather than turning them into ships intended to seek combat. Using an auxiliary aggressively against piracy or terrorism exposes a vital logistics asset to short-range threats such as rocket-propelled grenades. The distinction matters because losing a replenishment ship can constrain an entire naval force even when the encounter itself appears minor.