As per the release notes for 5.2a Beta 1:
"When adding tags to resources, common tags will now be suggested as you type. Existing user tags will be in bold."
This looks nice - an example is shown below:

It would still be nice to be able to "drag" tags from a listing of tags into a resource but this is a good and helpful step.
One of the benefits will be to help me inadvertently creating new tags by mistyping old ones!
Thanks Logos