I see what you're saying OP, but the balance in the retainer venture system IS the amount of time/expense of earning ventures in the first place. After all, if you choose to, you can have up to 4 retainers on each of your characters on one world....considering you can have 8 characters per world, that's a lot of ventures....but by making it so that it actually takes some effort to procure ventures, it eliminates the whole venture process being easy to do on 'storage alts' per se. You will naturally earn enough ventures doing normal in-game things to support your main character...and maybe one alt (if you play it occasionally as well), but you likely won't have a 32 retainer venture army at your disposal (I'm sure someone does...but that is basically all they do).

If you could send your retainers out w/o ventures....and/or if you could amass ventures even easier, it would create an imbalance in the system.