The reason it seems more off-topic/spam posts than the per-site Meta's is because it is the only meta site without a reputation barrier (despite the listed "[participate in meta][1]" privilege [see http://meta.stackexchange.com/questions/254054/remove-meta-privilege-for-mse-only for the reasoning]).

Anyone can ask a question here and it doesn't even require an existing account on another SE site.  Yes, this means it can lead to some spam and off-topic questions, but there are a number of benefits:

* It gives a voice to low rep users who cannot participate in their own site's child meta.  While the question will probably get closed since it only pertains to a specific site, they usually will get feedback via comments and answers before that happens.
* [Bug reports/feedback][2] from users without accounts

As for the Spam/Off-topic questions - how long does it usually last?  There is more than enough activity on the site to get spam nuked within minutes, and usually the off-topic questions get closed pretty quickly, and deleted not to long after that.

Echoing what [I said in my comment][3], if you turn MSE into a "High Rep Members Only" club, you only further isolate low reputation users from contributing to improving the Stack Exchange sites.


  [1]: http://meta.stackexchange.com/help/privileges/participate-in-meta
  [2]: http://meta.stackexchange.com/questions/153414/reputation-requirements-for-posting-on-meta-stack-exchange-and-per-site-metas#comment792729_242489
  [3]: http://meta.stackexchange.com/questions/260319/consider-using-migration-from-per-site-metas-as-only-path-to-meta-se-for-low-rep?noredirect=1#comment848692_260319