That iMessage-toggling is unfortunate, IMHO. When I travel internationally, I would like to pop in a local prepaid SIM obtaining voice+SMS+data and continue to receive iMessage's targeted to my original number whether I'm on WiFi or walking about relying on cellular data. I would go out on a limb and opine that the vast majority of iPhone owners in the general public -- i.e. excluding tech-savvy people, like on here -- are completely unaware that one may configure iMessage aliases by way of email addresses. As such, it certainly would be more convenient to me if people could easily reach me via iMessage and vice-versa using my old number and without loss of a message thread. WhatsApp is able to do that. After installing WhatsApp initially, my primary, U.S. number was associated with the app. And, so, when traveling internationally and swapping out my old SIM for a local, prepaid SIM, my friends who used WhatsApp could still contact me using my original phone number. I did not have to tell them my new, temporary phone number in another country to message me with WhatsApp. And since that's a data-based service, then giving my US phone number to the locals in another country doesn't create any long-distance charges for them to text (or even call me) via WhatsApp. And, finally, of course, when I return home and swap my US-based SIM back in, the foreign friends can continue to reach me using my original phone number.
In summary, when I travel internationally I do not want to incur any roaming phone, SMS or data charges. I want to use a temporary prepaid SIM in that country giving me a local ph# for cellular calls and SMS, and allow me to use data. I'd also like my friends back home in the US to be able to continue to reach me by iMessage using my original US phone number via Wifi or cellular-data. However, it doesn't seem that the dual SIM iPhone's will provide me with that. Instead, I think I'm just better off with a single SIM that I swap out for a temporary one, and use WhatsApp (or even, better, the more recent "Signal" app instead which avoids some of the concerns with the former by some owing to its acquisition by Facebook).