Generally speaking you get what you pay for. I race at a track that draws a very fierce competition of talented drivers and seen most of the vehicles and engines run. When I buy a vehicle number one thing I look for is durability! I admittedly am not a good driver and in order to get better I have to run a vehicle that stays together. After running my mugen mbx5t for 2 months 5 more people bought them cause they couldnt believe my truck stayed in one piece and didnt break! Some say Mugen should sponsor me not because Im a good driver but because I drive terrible and it is a testiment to mugens quality.
I have only seen one OCM buggy and it looked very nice but it broke numerous times. I have seen several caster buggies and while they are very much like a mugen they certainly didnt stay together like one. I saw drivers much better than myself carrying their broken buggy to the pits while I ran months with not one broken part.
I also see the same trend in engines. The makers like go tech(not to pick on them but just as a example) are very similar to a novarossi and are much cheaper in price. However they simply(from what I have seen) dont hold up in the long run like the novarossi.
As far as parts supply at local hobby shops to me that is a big thing. I am lucky and the hobbyshop I go to in Evansdale Iowa carries parts for all the major brands and has excellent service and as a extra bonus is cheaper than ANY online shop I have found yet on most kits,engines and parts. Now some people dont have a hobbyshop close and so they order online. I live 5 minutes from one hobbyshop and 20 minutes from another yet I spend my hard earned cash at a hobbyshop 1 hour away because of their great service,price and selection.
In the end I always tell people to buy what they like and what they can afford, but if you can afford mugen over caster and novarossi over go tech that is my recommendation as you will be happier when your stuff is reliable and not breaking.
Just to reinforce I am not flaming caster,ocm,go tech or any other brand I have mentioned, I just am answering the above question about why myself and others continue to support the major brands as apposed to some of the new companies coming out with copied/similar designs.