If you're a tourist visiting both Paris and London Eurostar wins hands down - town centre to town centre it's much quicker than flying.

For those of us living outside London, catching a train to St Pancras is, in most cases, much easier, quicker, more convenient and more relaxing than fighting traffic on the M25/M3/M4/whatever to get to an airport - and the cost of airport parking plus fuel will usually work out considerably more expensive than the train fare into London. (You mention driving to St Pancras... well, this is something we just wouldn't do!)

Flying to Paris makes sense if you're connecting, or if you live close to an airport, or if your destination in Paris is close to one of the airports.... but in all other cases it makes much more sense to take the train.