Why is it that you often start doing things in SharePoint that you find really useful, and when you have almost reached your goal – the whole idea just falls? This happened to me when I setup a taxonomy for a department, we built the trees and deployed them to the libraries. All looked really good and we should start adding files. And when you setup a new intranet, you usually have a lot of files you want to upload at the same time to the libraries. No worries, I told them to upload all files and we would add the metadata tags through the datasheet view. NOT! That did not work and we also got a readonly message. Embarassing… So well sorry, you will have to edit the files one by one. Well that is not a realistic scenario. Then the column filtering was messed up, it is not possible to filter on metadata tags that are cascaded. And also, in the datasheet view the values have got a number and a hashtag in front of the value.
And yes, I know that managed metadata is very useful for many other situations, but when it comes to using it in libraries where you need to frequently update metadata tags, then it is not useful.
Please send me all your argues to convince me that I am wrong!! Because I really like the concept of managed metadata.