Thumbnails were designed for the smaller screen of the iPhone. There's no reason not to use the easier to access tabs on a device with a bigger screen.
IMHO tabs rule. 1) It mimics how regular browsers work (more initiative to most people and 2) it's a more efficient way to surf. Personally, I hated the original iPad version of Safari.
I must admit when they updated to IOS5 I wasn't really sure about tabs. Put it this way it's less simple.
The first Ipad was easy to use the tabs are good but it adds more clutter to the screen same with notifications. I mean this is why Android tablets and phones are poor so many menus.
I think they should have kept an option to disable tabs if you don't want them. I had a few crashes with the old format on my Ipad 2 and tabs solved it however.
Icab mobile is good you can pick tabs or no tabs the downside is I don't feel it's quite as sharp as Safari but it's the best alternative. One thing those windows were not very attractive and it was very dated. But strangely it was very quick.
So yes I see your point and totally agree but I do like tabs aswell there's pros and cons to both.