I've noticed that many cake decorators in the USA use shortening in their buttercream to prime cakes before applying fondant. I haven't really come across it as much in the UK. Am I wrong? I was just wondering about preferences on this side of the pond and on the other.

I've always used butter as I'm in the UK and shortening isn't as prevalant or as cheap here.

I love the taste of buttercream using butter. After all, it's not called shortening cream! :D

A lot depends on which butter you use though. My first attempt was with Asda's won brand and it was......meh. I moved on to Stork and it was better, then I switched to Lurpak unsalted and never looked back. Works flawlessly all the time.

I always got the feeling that the buttercream wouldn't taste as nice with shortening?

UK and USA cakes and frostings can be very different.  Before the "cake shows" on TV, fondant was rare or unheard of in the USA.  Once fondant became the new rage, people simply used their regular frosting underneath.   Crisco was King until about 2012 when they switched from using trans-fat in their formula to zero trans-fat. 

Crisco with trans-fat was cheap, dependable, made great tasting "buttercream" that could pipe well, crust, and most importantly, produce consistent frosting that could survive super-hot/humid weather or the dead of winter. 

In 2012 Crisco was the first to see the future wave of "healthier eating" and did a preemptive strike and changed their formula to zero trans-fat.  Crisco withOUT trans-fat does not make great frosting; so people started adding butter or went back to all butter in their icing. 

Butter "buttercream" tastes great, but it is more expensive, and melts at an outdoor wedding in Texas in August.

(Even though fondant is the new rage, you still won't see  3" high mud cakes or fruit cakes with ganache as the base under fondant.  People in the USA are still used to the traditional taste of Betty Crocker with Crisco buttercream.)

Yes, a lot of people and bakeries in the US use shortening plus artificial butter flavor in their icing rather than butter, and when Crisco changed their formula many of them switched to store brands that still had trans fat, or  to high ratio shortening instead.  I love the taste of all butter buttercream, but can't use it in summer (even up in Michigan), and it tends to melt in my hand when I'm piping with it, so I compromise and use half shortening (store brand) and half butter, so it tastes better than the all shortening stuff, but is easier to work with than all butter.