personally i only have 1 extra retainer, its been enough for me to sustain materials and equips for all my crafters (all 50 btw) so yeah you can save yourself not 6 but 4 bucks a month, it doesn't hurt you and you gain something out of it or unless you like the idea that your presence in the fm with the extra retainers makes you a little bit more being able to sell more stuff, 3 is such a magical number

but yeah 2 extra retainer is kinda overkill imho yes theres lotsa material to be thrown in but how much of that stuff do you use to craft or sell and looking back its alotta stuff that needs to get thrown out (which btw SE needs to implement discard in the retainers inventory)