Is it just the body plan of the ancestral bilaterian and therefore remained prevalent due to absence of selective pressure against it?
You can see some animals which precede the emergence of this plan, like sponges. They apparently do alright, so there's nothing precluding different body plans. However sponges also have a fairly limited niche, and perhaps there's a good reason. Similar things can be said for other non-bilaterians like jellyfish.
One could speculate that the "tall-donut" shape is the key innovation. When food moves in only one direction, it is much easier for the digestive tract to specialize and digest food much more efficiently in stages. Animals, after all, are things that survive by eating things and digesting them - the faster they can digest, the faster they can eat, the better they can survive.
Animals which have no separate anus seem to have more trouble eating things and digesting them: It's slower, and they can't eat as varied a diet.
A worm, being essentially a long tube, is a simple way of accomplishing accomplishing the one-way GI tract. If you think in an evo-devo sense, it also doesn't take a lot to progress from a "basket" to a tube (the two layers after gastrulation just have to meet, fuse, and form a second hole). If you consider the so-called higher animals, like humans, cats and dogs "worms", then we can stop there. Otherwise, we can remark that the "worm" shape also allows easy movement in water and on soil by wriggling.
Past the "pure worm" you can see a large variety of body plans, such as among arthropods, but they're really just worms with a few specialized segments, including, say, humans. This does have a fundamental reason: During early development, patterning of the body (deciding which organ or limb will grow where) relies on Hox genes, which are typically top-level transcription factors that determine a segment's identity. So the "worm" also happens to be a nice developmental "base" from which more complicated embryos can develop.
is it a neutral adaptation? Or is it actually a positive one?
This is very hard to answer:
- There are necessarily many confounds, and we cannot easily find two animals which are otherwise the same, but one is a worm and the other is not. We cannot artificially generate such animals yet either.
- The obvious way to infer fitness is to look at whether the trait came to dominate the population. But with since body plan establishment is something that did not evolve independently many times, it is hard to say whether it dominated due to a genuine fitness advantage or purely out of being fixed. Perhaps there are other, yet more efficient body plans, but they have not yet arisen due to random chance.