I figure that Nintendo, like other game publishers, are more than willing to serialize a money-maker. Mario Party is a very interesting test case: glancing over the series rough performance over its iterations (via vgchartz, not perfect, I know) Nintendo was pushing out product annually on the GameCube, despite waning critical reception. The reason is as any would guess, the games sold very well and the development budget was likely relatively spartan compared to most of Nintendo's 'Cube efforts. With ROI like that, and remembering Nintendo's platform struggles during those times, who could blame 'em for capitalizing on what the public apparently wanted anyway?
If anything, I am now very surprised that they have only thrown 2 Mario Parties on the Wii...number 8 (released 2007) leveraged that wide "casual" Wii userbase for some spectacular sales! This would indeed support an argument that Nintendo is "better" than many of their 3rd parties when it comes to milking a series. "Better" sure is subjective tho; If it were me running Nintendo, I would have wanted at least one other New Super Mario Bros. on the Wii. (30 million presumably happy customers would likely be quite receptive...but Nintendo's unfortunate failures to adequately support their successful platform is another topic.)
Game companies that leverage a popular property with multiple sustaining releases is the logical tact, I don't think there is anything "wrong" with it perse. In fact, I wouldn't mind if they could "milk" a couple OG-style Metroid adventures outta themselves (listening to RFN's Super Metroid spectacular as I type)...a second NSMB for Wii would likely have paid for it and then some.