People's ear for music are very different from one to another. They consider the differences as the "purity" of their specific niche in the genre, which may be Pop or Rock or Country.
For example most metal bands would consider adding a fiddle player, or a banjo player as anathema. You don't see armies of percussionists in a country band. Even the somewhat milder notion of adding a keyboard player to some bands is viewed with disdain.
There are those that insist on maintaining the "purity" of the music that they play in their niche. In many instances what that means is that the only legitimate instruments that can be used are the guitar, bass and drums.
For me, I like contrast. I love being blown away by a rock-em, sock-em hard hitting number and then having it followed by the prettiest song I ever heard, and then followed by something musically intricate and interesting.
This could mean using other instruments besides the mere guitar, bass and drums.
I have a video of an old Eagles concert (one of their many "farewell tours"). Besides the Eagles themselves, they used multiple keyboards, percussion, and a horn section - one of which also played the fiddle. Consequently, they were able to pull off several different styles of music.
Santana uses an army of conga and timbale players in his music - talk about being musically interesting!
With Jethro Tull, it was the flute. With The Charlie Daniels Band, the fiddle. With Chicago, a superb horn section was featured. Who can forget "25 or 6 to 4", "Make Me Smile", or "Beginnings"? This was really good stuff.
Talk about a cool band -- How about Earth Wind and Fire?
And then there was the Electric Light Orchestra.
There was more - a lot more.
The ultimate for me was when Andrew Lloyd Webber came out with "Jesus Christ Superstar" which was a rock opera that blended rock with blues, jazz, folk, pop and classical music.
Music evolves and will continue to evolve. I think it's great that new ideas are tried with different types of instruments. It makes for more interesting music.
The guitar, bass and drums will always be the foundation of rock and roll. If you want to be a purist that's fine, but there's nothing wrong with others experimenting with new sounds and new instruments.
Bring it on!