I think your question from title is rhetoric ;P and your real question is about current reputation concept on Area51.
Actually this is not a problem of rep requirements to create a proposal - you need 50 rep to create one! The actual problem is, you get exactly that amount of rep upon registration! So the number of proposals you can create is limited only by your determination and resources (namely free time and money).
I think it is a problem. Some people are creating proposals for temporary events, such as Arab Spring or current situation on Ukraine. I think they were opening fora wherever possible to register, and Area51 was one of that things. There are also very many proposals, which pure sense is to try to earn some reputation for the OP (they are even sometimes not asking any example questions in order not to get downvoted, counting only for some 'sympathy follow').
I think that the amount of 'startup reputation' should not be enough to create proposal. Asking questions - yes, upvoting - not yet, they should first learn what is good and what is bad question.
Should we require some network reputation (like 200 on any site) before creating a proposal? I think it makes sense, it would be good, actually, to require some meta or moderating activity as well, but it's harder to measure. Since you need at least 100 people with 200 rep for proposal to reach beta phase, requiring the starter to be one of them isn't something restricting.
There should be also limit on creating proposals. 365 per year is much too much.