As a McGuffin, I have no problem with the Allspark. My issue with it is that the movie did a very poor job of communicating what the use of the Allspark was for either faction, what the Autobots intended to do with it (for much of the finale, the Autobots go through great lengths to protect it, which makes their decision to destroy it, while understandable, pretty sudden), or what the exact properties of the Allspark are insofar as they are relevant to the plot.It seems like Bounty is working really hard to make the All-Spark seem like bad writing. Yes, it's a MacGuffin, but that does not in itself make it bad writing.
The whole point of a McGuffin is that you have an item of great importance without putting any sort of backstory into, but that only works when you at least make it clear how that McGuffin relates to the protagonists. Transformers did a really poor job of making that clear in my opinion. You can piece it together, but when watching the movie it really feels as if the writers were making it up as they went along just so they could get their set-pieces in.
