If u want to install many Apps or don't want to run in out-of-space condition, then it's better to use sdcard_only + repartitioning.
Otherwise it's sufficient to use it with normal kernel + repartition .. and then it doesn't matter if u insert sdcard or not... while with sdcard_only it's mandatory to insert sdcard ... otherwise there's no usb storage at all ... that means camera app will crash and so on.
Other option is to use normal kernel + rapartiton .. and sdcard inserted and optional u can swap it .... but this variant doesn'T work 100% properly will al apps ... as some apps don't like it if the sdcard is swapped.
Anyway if the internal usb storage (sdcard0) gets full, then u can still move some apps to the real sdcard... in Settings ---> Apps.
In any case, i do suggest to repartition the internal flash memory because otherwise a lot of space is wasted and not used at all.
As u can see there are a lot of variants, it's your decision what will fit best your use case of the Phone.
And can we revert to normal repartition condition? If we on repartition condition?