Different tools for different situations as Mike Evans stated. Yes, they do require more skill to use, but they also aren't needed for every situation. I'd say more skill is required in knowing when to use these weapons over being able to use them at all. I'd also say there are more situations where Multis are just more efficient due to extended battles, for instance, RES hunting. Falling back to Multicannons is easy when you likely won't need Rails or Cannons unless you are PvPing, doing Assassinations (still questionable), or just specifically hunting big ships.
It all comes down to what you are doing and how long you are doing it for. That doesn't come down to skill in usage, it's about efficiency. I can use Rails just fine, but I don't, really no need for me, I do sometimes fit cannons though. There is always room for discussion about these things and I don't think anyone is demanding that anything be made easier here. Multicannons are a very solid choice due to their staying power in multiple engagements over a longer period of time, which is probably the situation a lot of people find themselves in while farming credits or Combat Rank.
I think these reasons are why people fall back to them and label them better. They are better *for them* in general play, unfortunately they then come here and try to turn it into a fact in their arguments for balance. That's why these discussions are good, we can find error in our ways.