The reason is that these are streets with lots of businesses/stores/etc on them. They didn't want to make them permit-only 24 hours, because that would stop non-residents from utilizing businesses, so they become flexible; residents can park overnight, non-residents can park during the day. It starts at 10am so that people from out of town don't park all day in order to commute from Somerville via bus and train (on the approximation that most people work at 9am) and it ends at 2am so that visitors/employees of night-oriented places don't have to move their cars before places close, but do not park all night. They are also exploring options on spur streets related to these corridors, and other streets and special circumstances, like Vernon Street studios, churches, etc.
As for the idea that "lets do something sucky, but so much less sucky than the completely insane and out of line thing we proposed at first, the it'll look okay and people can't complain because it's such a big improvement." I have to say as someone who worked on the parking task force you have it wrong. It is not about making things "look okay" or stopping complaints. People complain no matter what the plan, they complain about how things are now, how they might be, how they should be, about anything and everything. Nobody at these meetings was worried about eradicating complaints. And it's not about "doing something sucky." We had the impossible goal of balancing the needs of the city (i.e. revenue), the needs of residents, and the needs of businesses. Of course nobody gets exactly what they want, nobody is 100% happy, but we tried to make things better than the initial proposal, NOT to make things look a certain way or some conspiracy to fool people.
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As for the idea that "lets do something sucky, but so much less sucky than the completely insane and out of line thing we proposed at first, the it'll look okay and people can't complain because it's such a big improvement." I have to say as someone who worked on the parking task force you have it wrong. It is not about making things "look okay" or stopping complaints. People complain no matter what the plan, they complain about how things are now, how they might be, how they should be, about anything and everything. Nobody at these meetings was worried about eradicating complaints. And it's not about "doing something sucky." We had the impossible goal of balancing the needs of the city (i.e. revenue), the needs of residents, and the needs of businesses. Of course nobody gets exactly what they want, nobody is 100% happy, but we tried to make things better than the initial proposal, NOT to make things look a certain way or some conspiracy to fool people.