Much like Apple, Google isn't usually the first to do something, but rather they look at what others are doing and if they think that it can be done better they put out theirs to set a benchmark. The Nexus One was their answer to phones with slow laggy processors and no on board storage. The Nexus 7 was their answer to the Kindle Fire, Galaxy Tab 7 and an assortment of other 7 inch "craplet" (as Phil calls them). In both cases manufacturers were putting out mediocre products just to sell them. Google launched the Nexus 7 to serve as a platform for the Google Play re-branding and as a competitor to the Kindle Fire and Nook Tablet, as well as to set a minimum bar for other Android tablets. They showed you can do a high quality tablet and if you market it and price it well people will buy it. Up until then all of the 7" android tablets were pretty crappy.
The Nexus 10 appears to be an attempt to jump-start the stagnant 10" tablet market. With the exception of the Note 10.1 (and the Transformer with it's keyboard dock) there hasn't really been a compelling 10" tablet since the original Xoom. Jerry said in his review (and I totally agree now that i have one) that the Nexus 10 is a full generation ahead of the other 10" tablets on the market (I've had access to and used the Galaxy Tabs and Xyboards, and messed around with a couple of the Asus tablets). It's fast, lightweight, has great speakers, and an amazing (better than Retina) quality screen (and 2 solid cameras). This is how Google wants the next generation of tablets.
Right now I dont see them doing a "phablet". At this point it is more of a niche (a growing niche but a niche nonetheless). There have basically been 3 "phablets" of any consequence. The Dell Streak (notable because it was first),, the Note (and Note2), and the LG Vu (Intuition). Of these the only one selling in any real quantity is the Note series and both of them are beasts. Right now there is no need for Google to enter this market. If the niche continues to grow then I can see them releasing a "phablet" a year from now or so, but not now.
Don't confuse what you want with what makes sense for Google and Android.