I believe that you are correct that a local guide is required. It is not that the trails are so difficult. It is more of a government-sponsored work project.
For years, the regular route was called the Coca-Cola Route, because local teens would carry Coca-Cola in bottles up the trail and sell them at the various overnight camps. It is a little weird to be at 15,000 feet on your way up a big peak and there is some kid hawking soda outside your tent. Local guides liked it because they could hustle the tourist up and down the mountain in minimum time, yet they were getting paid by the trip. Lots of tourists get sick that way.
For ten years or so, there has been a move to get more tourists off that route and onto more diverse routes. They tend to be more scenic and a little longer. This means more porter-days, so this aids the government-sponsored aim.
If you are on a tight budget, you will hire your local guide when you get to Marangu, and then you will hope for the best. They just want to take your money. You might get better results by signing on with a stateside travel agency. It will cost a lot more, but the amenities may be a lot better, and you will feel safer.
I was there in June 2000 and went up the Shira Plateau-Western Breach Route. I think it was later closed due to unsafe rockfall conditions onto the Arrow Glacier. However, I felt like our route was slightly longer, more scenic, and preferable to the Coca-Cola Route. I enjoyed it, and it was one of the easiest big peaks that I have ever done. (I believe that this route has been re-opened).