I thought the round ones were fine.

What is the purpose of the spines?

On a tank sized beetle, they won't be to keep off inquisitive birds.

Most tyranids are spiky, so it may be in the default gene set.

If it's a true exoskeleton, they could be reinforced 'knots' of chitin that various muscles are anchored to.

They could be sensors or signals- pheremone, colour changing or pysker

Depending on how the Bio-tank weaponry works, they could be heat vents / cooling fins used to avoid the creature overheating.

In that case, rounded lumps may be more appropriate.

casting, help to painters and rule of cool are the significant factors here. fluff can be easily justified for any situation.

_________________
My shifting projects

Imho all spines on Tyranids are for breathing. They have holes on the tip.

_________________
We are returned!
http://www.epic-wargaming.de/

I've always assumed they were purely for looks.

_________________
ONSLAUGHT MINIATURES onslaughtmini.com

For whatever they are good...I find the spiky ones nicer. The round ones look not tyranidlike enough for my taste. So, keep it up. I am eager to see it with legs. And...I do like the horne

Who cares what they are for. Spiky bits look better.