In a recent session, my cleric was put under the control of a Suggestion spell and was directed to discard his shield, which bears a holy symbol. The enemy knew that clerics use holy symbols as a focus, and attempted to limit my ability to cast spells by removing the symbol.
Due to my background as an Acolyte, I was given a second holy symbol (an amulet) which I wear around my neck at all times. Once the Suggestion spell was broken, I argued that I could still cast spells without providing trivial material components by using the amulet instead of my shield as a focus. After the combat was over, I picked up my shield and began using it as the primary focus again.
Does anything in the rules prevent spellcasters from obtaining multiple spellcasting foci and switching which to use at will?