long and short vowels.

[gigas phoberos]

In Latin each vowel can be either long or short. And there's no way to predict whether a vowel is short or long, thus you must
memorize each word with its long or short vowels. But how necessary is this if your primary interest is reading text?

[modus.irrealis]

If I'm not mistaken, the Ecclesiastical pronunciation doesn't distinguish long and short vowels, and people who use it understand the text just fine. Although they do have to memorize where the stress goes (e.g. videre vs. facere), but that basically comes down to memorizing whether a single vowel is long or short (and only for certain kinds of words), so it's much easier. In fact, I assume that you could even ignore stress without hampering your ability to understand the text.

There are some words that distinguished only by vowel length, e.g. nominative fama, ablative famā or present venīmus vs. past vēnimus, but unless you're reading a text where macrons are used (and my experience is that these are rare outside of material for beginners), it'll be context that decides in this case both whether the vowel is short or long and which word it is.

On the other hand, if you do think that down the line you'll want to know the vowel lengths, it's much easier to learn them from the beginning, instead of learning them afterwards.

[metrodorus]

It depends on how you want to appreciate the text. Writers like Cicero put a LOT of effort into making their prose flow with a certain cadence - even the choice of words is determined by the rhythmic requirements of speech. If you don't have the sound of Latin in your head, you lose this, and so, you lose a degree of pleasure. Latin poetry also cannot really be appreciated in full, without an understanding of quantity.
So, it really depends on what you mean by 'reading'. If you learn your vocab orally, then you won't have to 'memorise' the macrons, the word will simply sound right one way, and wrong another way. You'll be able to read quite accurately without macrons.

You also can't get the accent in the right place in the ecclesiastic pronunciation, without knowing which vowels are long by nature, as this affects the placement of the accent.
Metro.