Why was the Logstash web ui discontinued?

if there is an issue with the data that logstash is receiving, wouldn't a monitoring page be of use? even if all it did was to tail the logstash error log file(s).

The Logstash web ui was only ever to visualize data that made it into Elasticsearch. The original Logstash web was replaced by Kibana 3, and that has now been superseded by Kibana 4. As Kibana 4 has its own server (as it is node.js based), the need to bundle it with Logstash (which provided the web server component for the bundled Kibana 3) became moot.

As far as monitoring goes, Logstash 2.0 will be API-driven, and monitoring will be possible. However, you will still likely need a tool like Kibana to visualize that data. Tools to visualize that data will not be bundled with Logstash, just as they are not bundled with Elasticsearch. We provide Marvel to aid in monitoring Elasticsearch, but you are free to use the API to pull the same stats and monitor them with the tool of your choice. The plan for Logstash is to do similarly.