No they are not. Measuring everything on the scale to a gram yields much better and consistent results than wondering if your dish will fail because some inexpedient was more tightly packed or if your spoons are the same spoons that someone else had.
And while a person could argue that ounce vs gram is moot, it is total nightmare for volume given stuff on dry ingredients.
Yet for part-time cooks as most of us are weighting is impractical: we don't have scales, and even if we do the amounts are usually too small. And stability is not necessary the goal :) For home cooks the usual task is to scale a recipe up or down and with cooking units the math is much simpler. Another thing about cooking units is that the number of different units helps to convey that engineering tolerance I've mentioned; the smaller the unit, the less the tolerance. When everything is expressed in one unit, you need to specify the tolerance explicitly.
Of course, nowadays the scales are very smart and a pleasure to use. But here's the thing: with smart equipment we don't need to bother about being metric. It's not a problem for smart scales to display the weight in any unit imaginable. I think there must be culinary apps that convert between weight- and volume-based units to suit everyone's tastes and they're either free or cost less than $5. The proponents of metric system claim it's simple. Maybe, but it's a simplicity of a typewriter compared with a modern typesetting program. Why would anyone with a smartphone care about this kind of simplicity?
For example, there's ISO 216 standard for paper sizes: A0, A1, A2, etc. The sizes form an interesting progression: each size is exactly 1/2 of the larger size. But how did they select the 1st size in the row, the A0? It's pretty interesting: the A0 size is exactly 1 square meter. I bet the designers of the standard though it would be be a feature, because the users (e.g. printers) will be able to use this fact to simplify their calculations; e.g. you need to print 1000 A4, you know it's 1/16 of A0 and the paper is 100g/sq.m., and you go from there. But I don't believe anyone does this kind of math nowadays; everyone has computers and printer jobs have much more variations in paper sizes and densities for this "simple" rule to be of practical use.
It really depends on the recipe. Some cookbooks, for example, recommend that measurements be as accurate as possible for baking. However, I doubt the majority of the worlds population use scales to measure ingredients when they cook. In fact, great cuisines from around the world have developed perfectly well without the use of scales for measuring ingredients. Can you honestly say 250g of chopped carrots is preferable to two medium-sized carrots, chopped?
