Is it a shameless money grab? Most definitely.
Is the new content as bad as you think? No.
The new planet types add a bit more variety. It has annoyed me since original Sins that there were so few types. Well, here they finally are years later - if you're willing to pay for them, that is. One could also, of course, download one of the numerous mods by talented individuals and get this and much more absolutely free.
The new specialization system does add a [thin] layer of strategy to the game. You are correct that there will mostly be an obvious better choice and that this 'choice' is therefore in fact not very real, but the depth of the economic considerations are fully on par with, for instance, the decision on whether and when to build trade ports or refineries. The depth of the local system-level Sins economy has always been light, this is just another, new 'light' decision to make, and so it does add some depth. It allows you to develop your empire's 'tall' rather than 'wide' to a greater degree (a trending topic in modern 4X, see: Civ V).
Also, it should be noted that this DLC adds over 40 new exploration/planetary feature-thingies, some of which use new code that modders really appreciate. Bleh.
... and then, you hit the nail perfectly on the head with this comment:
So, what did the DLC add? Less content and less mechanic changes and a lot of awfully beta changes to the game than a PATCH did during Sins 1 times. For 4-5 bucks.
- though I don't recall "Sins 1" as the rose-tinted 'glory days' you seem to do and I think if they had been so great in the first place, the problems the original game had wouldn't be present in the game still (laughable Kol Battleship? Check, still here years later. Broken pirates? Check, still here years later).
In contrast stands a game like Natural Selection 2 which has received great - no, amazing support. Constant patches tweaking balance, news feeds from developers, major free content updates and so on. All this from the moment the game was released and with no sign of ending any time soon. Sins: Rebellion? A patch around release, then a patch here half a year later timed to come out same time this DLC is released.