Theres nothing inherently good about spanning multiple genera in a single game, although some of my favorites have done that (Borderlands, Tales of Symphonia(that was kinda an action RPG but the action was a bit clunky). Then theres Civ IV which is very firmly in the TBS genera along with Advance Wars and Fire Emblem. I really wouldn't say i'd buy a game (or not) based on ANY of the reasons you've listed though. I can't even be neutral cause i do notice them but i don't normally fault of praise a game for mixing it up.
Out of the games you've mentioned I've only played spore. I have to say that it was a letdown and honestly the part that i found most fun was the cell stage. The creature stage was very much a letdown (all-though that may be because i'm studying Marine Biology and so know a lot about ecology and evolution). But I think the biggest fault spore had was that it did not allow you to stay at one stage or another. It was very clear that your goal was to evolve and become intelligent enough to destroy the universe(i mean that's why we all played, right? right?). And yet i just wanted to stay at the creature stage and make freaky things with unique ways of living and see how good they were at living. The game i got wasn't at all what i expected, the creature stage was like an RPG, with clear advantages to using this over this, there was very rarely any disadvantage, only upgrades.
I don't know, maybe you could argue that spore was simply not done well, maybe you could argue that it was fundamentally flawed for using (or trying to use) many different genera in a single game. I would argue the former.
There are other problems though, you'll need to define exactly what each genera means if you want to do this thing right. I noticed that Portal 2 is listed as "Action, Adventure" (in the steam store) when the pre-order opened up yesterday, my first reaction was "not Puzzle?". Then you have games like Hitman listed as only "Action". I don't think that i need to point out how VERY different these two "Action" games are (but i will). But i would call Portal a Puzzle game and Hitman is a sort of mix between Puzzle, FPS, and Stealth gameplay. Both games are excellent (talking about the original Portal now) but they require VERY different play styles and so appeal to two different types of players that may or may not overlap.
*EDIT* I just noticed that the steam store makes no distinction between the kind of gameplay you can expect when it categorizes it in a genera. As they are listed, the genera, according to the steam store are Action, Adventure, Strategy, RPG, Indie, Massively Multilayer, Casual, Family Games, Simulation, Racing, and Sports.
So yeah theres a mix of terms that describe the gameplay you can expect (Action, Racing...) and some that describe the theme you can expect it in (Massively Multilayer, Family Games). But as i mentioned before, there are a lot more genera than the steam store (as a credible source (i mean valve has made some great games, surely they have some clout when saying what genera they think a game falls into) mentions, and lots of significantly different games within the ones given that will appeal to significantly different groups of people.
Now that i think about it though, i guess i'm ultimately indifferent to weather or not a game mixes it up genera wise, i care much more what the end result looks like.