botany - Evolution theory - roses spikes - being more bulgy doesn't give you advantage

I've seen spike, huge spike. And I thought that development of such spikes could be contrary to the evolution theory.

Being „little more” spiky doesn’t give you any advantage... So those individuals having one won't have smaller mortality rate than those with smooth stem. Even more, the individuals with bulgy (more spiky) waste resources their ruggedness and have more things to feed, which should mean that they would have higher mortality rate.

So I can see no way to develop spikes on the stems of roses according to the evolution theory which embrace "small improvement". Spike is only efficient (to deter aggressor) when it's very sharp, it's either sharp or it isn't - there is little in between.

Could anybody tell me that this is bullshit (and why it is bullshit), and that I should us believe in Darwin competence?;)