Blocking is probably the single most important role of a TE. Allen wasn't going to get a ton of targets at the best of times because Gronk was on the roster, if you wanted to throw to a TE, you'd get Gronk into the game.
IU'm 100% convinced that Allen's #1 job as TE2 was to take some of the physical pressure off Gronk. I think it is a masterful testament to Allen's effectiveness in a Patriots uniform that Gronkowski, who missed a ton of games before he got here, has been much healthier in the Dwayne Allen era and has never been too injured to play in the playoffs since Allen's arrival.
Allen was neither overpaid nor overrated. Allen's job was to ensure that Gronkowski stayed on the field and he did that job brilliantly