Sorry to resurrect but I did a google search and found this.
I really wanted Rivet for my mini but the damned dock icon would annoy the other half (her mini is a part time server, since she rarely uses it hehe) however I noticed one thing.
They say that Rivet has to have a dock icon for App Store rules, however, Alfred and Caffeine which were both downloaded from the store, run in the notification area and do not require dock icons.......
Bit of a white lie from TLAF perhaps?
Went with iSedora instead, and besides, Rivet aint even in the App Store yet either! So they've forced a permanent dock icon on everyone for no reason!
I was thinking the same - I replaced Evernote with the version from the App Store (so I get update notifications) and it works exactly like it did before with an icon in the notification bar and nothing in my dock
on the upper right and you will find "App Store" under Software Updates. That's a good place for it. Of course Apple will first park in the dock to encourage impulse buying.