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