I've survived using 2 retainers for a whole year before I bought an extra retainer. After collecting too many weapons and clothes, I was forced to get an extra retainer. I only use the extra retainer to hold my old gear since I don't like to throw them away and I like to use them for new classes or jobs I will be leveling. It is very hard trying to balance Jobs & Crafter gears & tools, plus material used in crafting with so limited storage space. I just wish I could swap items from retainer to retainer without using my character inventory space.

In the end, the extra retainers are useful depending on how & what you play. If you're a collector or a crafter, you need space. If you get gear and once it's outdated you turn it into materia and meld it to current gear, then maybe not so much since your getting rid of clutter that much quicker.