I used them alot in original P3. Igor tells you about them early on. Or maybe you have to have a Persona that carries one before he tells you about them.

Yeah, random encounters are usually much more of a problem, especially because those SP restoring items are so hard to come across. I made it a point to always get the player advantage, or just let my teammates do the dirty work in order to avoid them if they catch me. It is pretty much up to luck if they ambush you, especially if it's four enemies spamming instant death spells.