I agree with the previous comments about workload, meandering flightpath and tendency to fly heads-up. As a green cross-country pilot, I'm already pretty busy with just 'aviate' and 'navigate'!
There's a cultural aspect, though: not only do glider pilots tend to dislike driving around on instruments (conversely I've flown with some earlyish PPLs who spend far too much time heads-down, especially on hazy days), but in their early years at least they have virtually no contact with R/T operation. Unless you fly from a general aviation airfield, or there are particular local considerations, you'll probably not need a radio until you've flown your Silver C cross-country - a good couple of years or so (rather more in my case). Even then, a lot of pilots will rarely venture more than 10-20 miles from their home airfield, and if there are busy areas, they'll simply avoid them. Only the pundits will find much use for the radio... and in competitions, courtesy calls to the local FIS won't be popular if they distract you from soaring or let the opposition know too much about your movements.
When it came to renew my R/T licence a year or so ago, I was asked for logbook evidence of vhf usage - something that we don't record in the standard BGA logbook. All was duly sorted out very smoothly, but it suggested that the radio licencing folk at the CAA don't deal with many glider pilots as such. On asking around at the airfield, I found that few glider pilots have R/T licences unless they also have a PPL.
I got my licence because it seemed useful, interesting and vaguely responsible and, as I've lately been flying from a licenced airfield, it has in the end proved handy. Apart from the routine circuit calls, though, I barely use it enough to stay in proper practice. Renewal isn't cheap - £57 last time. The initial course and licence cost rather more, and the test itself is largely concerned with situations that are completely unfamiliar to glider pilots.
So here's a question: assuming that (unless I win the lottery...) I only make half a dozen cross-country flights a year on my very slow path to punditry, should I really distract myself and everyone else with ropey courtesy calls when I'm near controlled airspace? If so, how near and how often? Remember that while we can give an intended course, we may depart from it by miles, hours and thousands of feet; alternatively, if we're all to call in updates every ten miles or so, what'll happen on a sunny summer's day in Oxfordshire? And how well DO we show up on radar, by the way?
If advisory calls will help slow the creeping expansion of controlled airspace then I guess I'm pleased to comply ... but in the end they are called Visual Flight Rules.
Tim