Rocks/gravel on the bottom is (for the most part) an aesthetic choice. Does it give additional surface area for beneficial bacteria to colonize on? Well, yes, but the bacteria will colonize on a liner or other pond surfaces as well. So to answer your question directly - No. It is not **necessary** (as is also evidenced by the above 'no gravel for me' answers from successful pond keepers).
That being said, I will never have a pond without gravel on the bottom. I love the look and do not in any way find it to increase my maintenance or be problematic when it comes to keeping things clean. I will admit that in my shallow upper pool area where there are no fish, some 'muck' can & does accumulate. It doesn't bother me much and gives the tadpoles something to munch on. In the main body of my pond where the fish live I don't ever get 'muck'. It simply doesn't build up. I do, obviously, have to use a net from time to time & scoop out large debris/leaves that might fall/blow in, but that's simple to do & the gravel doesn't hinder the process at all. I never vacuum it. My fish do spend considerable time munching around on the bottom, rearranging the gravel & stirring it up. Perhaps that's why nothing ever accumulates? I dunno, but here's a recent pic of some of my 'cleaning crew' and you can see below them what my gravel looks like. But if it's not working for you, then yeah, change things up a bit!
