I think a lot of the appeal of Flexiroam lies in usage.
For example, I do a lot of business travel all over the world. It doesn't matter if I'm in a 3 hour layover in Heathrow, Frankfurt, Mexico City, Tokyo or Singapore - I'm going to be able to turn my phone on and it just works.
Next week, I'll be in Shanghai for 36 hours, with layovers in Frankfurt and Vienna. Sure, I COULD pay $10 a day to roam... or I could buy a local SIM, but then, am I going to buy a SIM for Germany and another one for Austria? And am I going to take time away from my clients - who are meeting me at the airport - to go shopping for a local SIM, for what is basically a 1 day stay?
The SIM is always with me, I never have to worry about losing it, I don't need to worry about pre-paying for a local SIM and only using half... it just works. It's awesome.
Now, all that being said, I've since switched to Google Fi which is even MORE impressive, but if you're not prepared to do that, Flexiroam really can't be beat for ease.
Now, if I was going to stay for a week or more in one particular destination? Sure. Get a local SIM. Absolutely. But Flexiroam means you don't HAVE to.