The only reasons why you would want to keep spot colors when rasterizing an image in Adobe Illustrator are:
- If you print in spot colors (1-2-3 colors only) or CMYK + Pantones
- Or... you want to quickly import these color mix into your publishing
software and don't mind merging the spots later
The reasons why you would not want to keep the spot colors:
- Your project will be printed in CMYK
- Peace of mind; no risk to forget a spot on a CMYK job
It's more a "unique circumstance" to keep the spots actually, since most print job (in general) are either grayscale or CMYK. But keeping the spot is quite a nice shortcut if your project is printed in Pantones only.
I just don't see why you would rasterize a vector if it's supposed to go for printing, unless you used some transparency or filter; the vector offers the best quality possible. And the pictures (photos) should probably have their Pantones and spot applied in Photoshop; honestly it may work in weird ways in Illustrator, I just don't do it this way!
There's not much reason to not convert your colors to RGB or CMYK when you rasterize them. If you're preparing a project that's supposed to be printed in CMYK, you're actually saving time by ignoring the spot and merging them with the Cyan-Magenta-Yellow-blacK separations. You also avoid making a mistake by forgetting this detail at the last minute, as it's commonly the case in print-ready files.
Just in case you didn't know, you can have special swatches of colors that use Pantones, but they don't need to be "spot." And if you work with spot colors and plan to print in CMYK, you'll need to make sure they are not spot anymore on your final print-ready file.
By the way, mine is checked by default. So that must be due to a preference setting somewhere.