The 18 hour ventures is just something to send your retainers on when one is asleep and/or at work, and also it offers a chance a finding something a little more rare. Sure, it would be nice to get special, super rare items more often, but then they wouldn't exactly be special anymore, would they? It'd be all, "oh look, my 4th fat cat/mine cart/littlefoot this week. Since the board is flooded with em, and all my firends have one, i might as well sell it to the vendor."

I will say that the 15 18 hour ventures challenge log entry is excessive, though. If anything, the two should be switched, with it only requiring you to do 10 18 hour ventures a week and 15 total ventures. My two cents.