[{quoted}](name=Rammus Soup,realm=NA,application-id=3ErqAdtq,discussion-id=Rc2EBIBi,comment-id=000800000000,timestamp=2015-12-26T04:08:07.933+0000)
I think you forget the added gold for a basic attack against enemy champions. After a short CD, spellthief grants you gold by simply clicking your opponent. Spellthief is great for passive farming, but it's even better for active farming. When it comes down to it, time is your greatest enemy. The faster you can farm and level, the better. Getting quick farm will allow you to overpower your opponent after the first back.
That "short cooldown" is fairly significant if you're continuously farming. Every time you kill a minion, it's reset to 12 seconds where Tribute is disabled. While there's a wave of minions, you will not be making use of Spellthief's Tribute.
If you are being activate with trades, it depends on which enemy Champion you're facing, because as mentioned above they will have the raw stats ahead of you if they started Doran's vs. your Spellthief.