gareth
its all down to costs, in the r22 or other piston the costs are lower but if you wanted to learn on a turbine there would be no problem however on a 135 as its a twin there may be problems depends where in the world you train
i would expect that the costs to train in a 135 would be in the region of 12 times the cost of a r22 and if you could afford to pay that much money you may find that being a commercial pilot would not bring enough cash rewards
best of luck with your training you will now get everyone elses views on this and we dont all agree all the time but we may on this ?
steve