Bear with me.
The way I look about this is: the more you want to be general about things, the less frequent definite articles are.
Scheepvaart is, en was, noodzakelijk voor handel.
That sentence seems just fine to me. However,
? De scheepvaat is altijd belangrijk geweest voor ons.
... is, as how see it, not the most optimal solution: I'd automatically go for the no-article form.
Now I've been looking this up in some of my grammar books from university (studying Applied Linguistics) and it's a problem that is, as it seems, completely dodged even by grammar books for Dutch speakers. My Italian grammar doesn't. It has a very articulate (heh) chapter about articles, and when or not to actually employ them. Interestingly, the Dutch doesn't have it. Now I found an interesting point of view for Italian definite articles that MIGHT be useful for Dutch. Proceed with extreme caution, but I believe it makes sense to say it because it fits in "my" view of the Dutch language. I'd need confirmation of other native speakers though, especially seen as I've returned yesterday from a five month long stay in Italy.
Anyway, here goes.
Italian determines substantives with a definite article (or something that "defines" the substantives in another way, such as a possessive "your" or a diesis "that") quite often, but it determines it more if after the substantive, some extra information follows or is implied.
Italian examples:
Il gatto "the cat" Quel gatto, che gioca nel giardino "the cat, who plays in the garden" litterally "thatcat, that plays in the garden"
Vado in pizzeria "I'm going to a/the pizzeria" (determination absent) Vado alla pizzeria di Marco "I'm going to Marco's pizzeria" litt. "I'm going to the pizzeria of Marco. Vado alla pizzeria. "I'm going to the pizzeria", implying [di Marco]. The speaker assumes that his conversation partner knows what he's talking about. If he doesn't, the speaker is forced to use the first expression.
Now let's see how this MIGHT be applied to Dutch.
Scheepvaart is altijd belangrijk geweest voor ons.
This is about as generic as you could get. It's not a specific scheepvaart, it's not THE scheepvaart. It's the collective scheepvaart, the sum of all the non-determined scheepvaarten.
De scheepvaart is, en was, noodzakelijk voor de handel.
This is more determined because of voor de handel. Now, as for the article before handel, that's because there's a preposition. I can't immediately think of any examples where a substantive after a preposition doesn't take the definite article: voor de handel, met de regering, door de regen, langs de rivier. Back to topic: because voor de handel is determining the "de scheepvaart" "more", you'd go for the article. For example, you wouldn't say:
De scheepvaart is, en was, noodzakelijk.
... because then, people would ask you what scheepvaart? What's specific about this scheepvaart? However, in your example, the answer is: "it's special, because it's related to trading and commerce." and therefore takes the article.
All that being said, "Scheepvaart is, en was, noodzakelijk voor de handel" would be, in my opinion, just fine because you can always pretend "every" scheepvaart is important and not just "any" scheepvaart.
I think we can conclude from this that Dutch's usage of the definite article is a bit uncertain, especially considering that even my uni grammars don't even mention it.
If my rambling could get any confirmation (or refutation!) from native speakers, that'd be great, because this is truly an interesting subject.