Yes Spotlight can search items once they're plugged in/inserted quite quickly, however, that's missing the point. The ability to search items without connecting them is much more useful and quicker.
Both methods find your files, but using the Finder is "Just not the same."
Yes, of course iView has the ability to search its own thumbnail database regardless of where the media is stored.
Having found your image though, you then have to attach or insert the media to do anything with it.
I am not saying iView is a poor choice as an app. I have used it, and managed a creative team that used it. What I would recommend though, is that potential users carefully consider their choices. Applications such as iView are not inexpensive and introduce yet another set of processes to be learned. Therefore, I suggest carefully thinking through your needs and options.
I am not a professional photographer, but I do have many thousands of images I need to access quickly, and which need to be backed up securely.
Once I have downloaded my images I batch rename and keyword them in Bridge. I am very careful regarding the file names and keywords. The process takes a few minutes. Then I transfer them to an external drive, my working drive. This drive is automatically backed up to another drive. When I want to find an image I search through bridge.
I can also search in the finder using the Spotlight process, save as a Smart Folder and burn to a CD or DVD, all very fast. I can quickly point Bridge at the Smart Folder to view the images in more detail, or I can simply run a slideshow right in Finder.
In addition to my working drive I have several other drives, all full of images. They are connected to my computer via Firewire hub. Therefore Bridge and / or Finder can search all my drives at any time.
I feel that over the years we have been 'trained' to expect that specific, powerful applications will always be required to run important tasks. The introduction of effective system searches is changing the field and requires a different way of thinking. I know a number of pros who now use their own folder system and either the OS or relatively simple applications for managing their library. A cricial element is that they apply a lot of thought and discipline to folder / file names and keywords.
I am sure iView and its competitors have other avluable features, but I really suggest those who do not yet have these appl;ications consider whether or not they really need the cost and learning curve.