Why Macrons Matter
There are some obvious places where macrons matter, e.g. the ablative singular of the 1st declension (Rōma versus Rōmā), infinitives in –ere versus –ēre, the future perfect –eris versus the perfect subjunctive –erīs, and so forth. Sometimes they help distinguish similar looking words, like peritus versus perītus, os versus ōs, and cecidī versus cecīdī. In the latter case, the ī is the result of monophthongization, so there’s more to it than simply pronunciation (and accentuation). For instance, lengthened grade is a marker of the perfect (faciō : fēcī; capiō : cēpī).
Of course, English no longer distinguishes vowel quantities, and we hardly pronounce Latin accurately. Thus, we usually pronounce tibi /tībī/ not /tibi/; /femina/ and /fēmina/ mean the same thing to us: “woman.” Still, it is an intrinsic feature of the language and made a real difference to its speakers. It is helpful to know where long vowels exist for purposes of scansion, and there are occasional other instances in which it is good, or necessary, to know them.
Macrons can be a pain to learn, but it is much worse having to learn them later, after already learning vocabulary. So I tell students from the start simply to consider them parts of the word. They mark distinct phonemes, even if the graphemes look the same. I illustrate this with the imperfect English example, “lava,” pronounced roughly /lāva/ (thanks, Brian McCarthy, for this). The word is unrecognizable if we flip the quantities: /lavā/.
In short, all macrons matter, even if some matter more than others. Students who learn macrons rigorously will find macron-less texts jarring, but there are genuine practical reasons for learning them.
Macrons and Grading
When grading, I tally all incorrect macrons and deduct that number from the final grade. In other words:
All forms are correct, but there are two wrong macrons (the missing one from amō and the one erroneously added to amat). So the forms are 6/6 = 100%. I will deduct two points for the macrons = 98%. This way, wrong macrons don’t hurt that much, but students still see that they matter. (Note: whatever scenario would have me quiz just on the present active indicative of amō is likely one where macrons matter much more, e.g. on a Chapter 1 quiz. In that particular case, I might only give half credit for amo and amāt. Otherwise, the example above is meant simply to illustrate the grading system.)
You may find that a student mistakes a macron on the root throughout:
In this case, I might only deduct one point for all six macrons (or three points instead of the full six), if it is really just a mistake learning the root (the student learned āmā- instead of amā-). That, though, is one of the few instances in which I go easy on mistaken macrons.