First of all no engine NEEDS to be soft mounted.... they all run fine hard mounted.

The issues are that of vibration which is a variable depending on what motor you are talking about and what type of airframe.

The smaller engines / plane suffer less.

Noise is a important factor yes............ but thats not all...
Aside from the airframe vibration you FEEL, there is vibration you dont feel.
Aside from Noise there is the matter of preservation of the airframe and more importantly (and often forgotton) the radio - particularly the servos... the poor dears are attached to two surfaces, one moves around alot and everythings happeneing, Im cold ... boo hoo I want to go home..... zzzzzip.. the gears go (Hitec only ! tee hee)

Arguably on a model like the C50 and similar sized models there is a limited gain given the avergae lifespan of the model and gear used. But lightweight soft mounting is possible without weight or wallet penalties. (thats what Im working on at the mo for IH)

Take the 2M models.... Forget noise... hard mount a YS140 (for example) as pure a beast as it is thats a lot of direct vibration throuogh the airframe and quite honestly is just RUDE !
Taken to this extreme your servos would probably be goosed in a very short time ! (goosed does not necessarily mean non operational... they might just adopt the precision and accuracy and sloppy centres of a Hitec !).

Sorry... Im being anti Hitec..
SLAP [X(]

You get my point... I hope..

PS - Hoping to Test the Fun Fly Wing this weekend