Hey Mohammad,
Thanks for your suggestion. I considered this while implementing bulk update but I decided against it.
In my opinion you should be very explicit about the fields you wish to update, due to the way that bulk update works it can generate very large and expensive queries if a lot of columns are updated, so it's better to be upfront about what you want to update rather than potentially having s situation where adding a large text field suddenly causes performance issues (or even database errors) anywhere you use bulk update with that model.