I think that the most important point has been given by CactusWoman, when (s)he says "Just because you cannot forsee a use for more "bulgy" spikes doesn't mean there isn't a use for them."
In addition to that, I would like to say that several plants evolved traits that were beneficial at a time where other herbivores existed and that are now useless. Those lineages did not lose their traits (yet?) just because there is a relatively low cost to making the trait in question.
Your post shows several of the very common misconception about evolutionary processes. Evolution is not all about Natural Selection. It is more interesting than that.
The conception that evolution proceed through "small improvements" is either partially wrong or totally wrong.
First, because of the loaded term "improvement". Evolution can yield a population to have a lower mean reproductive success or a lower complexity, things that you could eventually see as deterioration.
Second, because changes not need to necessarily be small. Some mutations have a very big effect.
Third, because even small deleterious mutations can spread in the population and reach fixation (=and all individuals carry this mutation), just because of the joint effect of other evolutionary processes such as genetic drift. This post may help you to get a quick grasp of what genetic drift is. There are already multiple posts that emphasize the importance of other processes than natural selection in evolution to layman. Here is one of them.
believe in Darwin competence
Evolutionary biology is not anymore
Darwin's evolution theory. It is way more than that. We've been working on the subject a lot since Darwin and today we have a much much better understanding of evolutionary processes than Darwin at its time. Everything is not fully understood yet and therefore there are still many people researching to better understand evolution. Of course, not to better test if evolution is true as this has been known for a long time now.