As the largest city in the uk without its own airport
.....don't forget Hull and Stoke, both are a similar size to Plymouth.
Getting back on topic guys.
It is on topic. PLH gets a mention in the National Connectivity Task Force report as a potential airport to have links to/from LHR if a third rwy is built. Had it not done so, it is unlikely that this thread would have been started.
The topics of ring-fencing slots for this purpose, PSOs, etc., are raised in it.
Heathrow Harry's misquoting of me also has a bearing on this.