To what extent are (non-urgent) PIREPs used by meteorologists?
This NOAA newsletter lists a number of specific ways that PIREPs are used by meteorologists:


To "assess the strength and position of the jet stream"
For thunderstorm and snow level forecasting
For forecast discussions of regional weather phenomena, like "downslope wind storms in the lee of the Sierra Nevada"
To "determine the likelihood of clouds thinning or burning off"


Their summary is:


  Better reports and observations, not only from PIREPS but also storm
  spotters, Webcams and road weather networks help improve the picture
  of what’s going on in the atmosphere. Automated temperature, wind and
  humidity reports from MDCRS and the Aircraft Communications Addressing
  and Reporting System in commercial aircraft also get incorporated into computer models. All of these observations lead to improved forecasts and warnings.


There are a few very generic references out there too. A 2014 edition of FAA Safety Briefing says:


  The information will then be added to the distribution system to brief
  other pilots, help generate inflight advisories like AIRMETs and
  SIGMETs, as well as updates to the Current and Forecast Icing Products
  (CIP/FIP), and improve overall forecast accuracy.


And an AOPA article has a brief mention:


  when the Aviation Weather Center (AWC) receives a pirep,
  meteorologists use it to verify that their weather forecast is correct
  or amend it if the pirep contradicts the forecast.


Finally, PIREPs (and AIREPs) are shown on the NOAA website, although that probably isn't what you had in mind by meteorologists 'using' the information.