I assume that we are talking about a modular IR course and not integrated. On the latter, if the school has an FNPT2, students will do 40 hours of sim time in that device plus some on a Frasca during their single engine training.
One reason that schools offer a syllabus with more than 50% of traing time on a sim is to try and reduce the cost to the customer and thereby make the course more marketable. As an instructor I do not entirely agree with the philosophy, but in this cut throat world in which accountants rule it does make a certain amount of sense.
As to how useful it is (or not), I have to agree with carbonfibre in that it is the quality of the instruction that really counts. BraceBrace hit the nail on the head talking about the ability to freeze a situation, debrief it, rebrief it and the reposition to refly it.
I may have missed something in the posts on this subject, but I get the impression that in many cases subscribers are talking about doing all the sim time before starting on an aeroplane. In this case I heartily agree that it can be counter productive. However, if the syllabus is properly conceived then flying and sim time should be integrated. Initial training (10 hours perhaps) on the sim to learn procedures and revise instrument scan, then a mix (2 to 1 sim v flying for example) till near the end then the final run up to the IRT in the aircraft.
One other reason for all this sim time has not been mentioned. If we are talking about modular training, it is highly likely that a student will have done very little IF recently, (apart from the basic stuff required for the CPL Skill Test). It is equally likely, as I have seen at first hand, that the student may never have flown a departure, hold or instrument approach before. I suggest that this is not the time to learn these skills at aircraft hire rates. By the way, I wish to state here and now that I am not trying to belittle anybody going the modular route.
In summary, a good sim, especially if it is an FNPT2 that accurately represents the aircraft, is an extremely useful and money saving device. It has many features that no aircraft can possibly have, and if used in conjunction with a good instructor, is well worth spending all that time in.