He said that satellite speakers are specifically made for the job of providing clear crisp detail whereas the Wharfedales (and presumably most other speakers) are designed to offer a full range of sounds, from deep base to high treble. He said detail would be lost if I used a full range speaker
Trying to give the salesman the benefit of the doubt, he may have meant that the rears in a 7.1 system get so little signal, that some speaker capability would be wasted.
As my system evolved, I also moved speakers back and ended up with full range towers as both side surrounds and rear surrounds in my 7.2 system. Frankly, I think the speakers have much more capability than they use. Side surrounds don't get a lot of signal, and rear surrounds get even less.
In fact, I've thought about getting rid of those 4 towers and going to smaller bookshelf/satellite speakers for surrounds.
The only time I see a benefit is the rare occasion when I use All Channel Stereo for music to fill the room.
The advantage towers can have over "satellites" is bass. If you have a good subwoofer or two, bass extension to 20Hz or below is not necessary in your surrounds. And for the money you're talking about, you won't get good bass anyway.
Generally speaking, 2 speakers for the same money, one a tower and one a bookshelf, the bookshelf will have a better mid and tweeter. The tower has to use a cheaper mid/tweeter to include a woofer. Maybe this is what the salesman meant.