Disco probably moved the common DJ up a few notches, and things have somewhat spiraled out of control since then. Nowadays some DJs are world superstars who command huge sums of money to spin for an hour in exotic clubs around the world, living like rock stars used to, and buying mansions in the world's biggest cities, but of course that is only a select few.
Just playing music is not good enough LED Light Pro Guys the modern turntable maestro has to learn their craft, like learning a musical instrument (although I definitely draw the line at calling them musicians!). To be good and get noticed means hours of practice and becoming technically proficient, it really is almost an art form, and to watch someone who knows what they are doing is great, but you also need to be a showman to really get the crowd pumped and into it.
Some ingenious companies have spotted this hole in the market and are producing quality lights and effects designed specially for the amateur, but with pro results. The price of things like foggers, LED systems and color-balls, all with sound to light features, have dropped tremendously, and now the average DJ can show up with a pro looking rig and put on a decent show, without breaking the bank.
I think most people have a vision of 4 multicolored lights on the front of a table when you mention semi-pro lighting systems, but technology has enabled the cost of manufacture of these items to fall dramatically and the quality has stayed high, these are not kids toys, and they really do make the user look like they mean business when they show up.