How do you "win" by having multiple Retainers available to Venture? Yes, you'll get more items - but, as far as we know, Retainers gather very few items even at their highest level. You'd also need gear for every single retainer, as I assume it's equipable and once attached to a Retainer another one can't wear it at the same time. It'd take a good amount of time/effort to actually maximize the effects of all your Retainers (you'd also have to level them ALL up), so in reality you aren't paying to win - you're paying to get access to the ability to put in an (at least) moderate amount of effort for very minimal reward. There are a lot of factors we really just don't know about in regards to Ventures so it's kind of hard to defend or be against it... though I can't really imagine gil will make you "win" anything in a game where the best gear is from dungeons. Maybe you can monopolize some area of the market with enough work (and assuming you're gathering/crafting while all your Retainers are... among many other things) but that's about all you can really do.

Paying for more Retainers doesn't seem like that big of a deal to me. I probably wouldn't bother getting any unless the price was low and I was running out of inventory space - not the case for me atm, however.