One thing which needs to be appreciated is that to fly around Europe in controlled airspace under IFR, one needs more than an "IR".
One also needs an appropriately powered and equipped plane. The minimum altitudes at which one can file Eurocontrol routings are about FL070. However in many areas FL070 results in a terribly roundabout routing and FL100 is where it starts to be viable. FL120 is required in many places to avoid big doglegs. FL140 is pretty good, and is the minimum to cross the Pyrenees. FL160 is required to cross the Alps. There is a lot of detail I am leaving out here (like minimum LTMA overflight levels being about FL100 plus) but basically that is the picture. The routes cannot generally be worked out from an airway chart; one has to use special routing tools to develop them.
So you need a capable plane, very preferably with oxygen, etc.
Above FL095 (and airways routings don't work well below that in many places) the only legal means of navigation (in GA context) is an IFR BRNAV approved GPS installation.
Mode S of course, everywhere.
I don't think many people have access to this level of equipment. In general you will be an owner, or in a reasonably upmarket syndicate. Then there are the Cirrus "zero equity" rental groups, etc. but they are far from cheap.
There is also a significant amount of technical/operational knowledge, much of which isn't taught anywhere. But that's another story.
The IMCR, with its restrictions, suits an awful lot of casual flyers/ renters very well.
The IMCR training is also generally used towards the FAA IR, although last time I wrote about this I got hit with a filthy libellious and widely circulated email alleging flying on a fake IR etc. In practice this is usually moot as almost everybody reaching the FAA checkride standard will have done the whole FAA 15hrs dual time in the USA with an FAA instructor - myself included
I think most FAA IR holders started life as IMCR holders and this enabled them to do an awful lot of instrument practice on their own, which is much more relaxed as well as cheaper than flying with an instructor as one would for 50-55hrs for the JAA IR. So even if they did not strictly speaking use their IMCR training time towards the FAA IR's 15hrs requirement, they still saved a big pile of money. By the time I went for the FAA IR, I had a bigger part of 100 instrument hours already.