My opinion doesn't really matter, as I'm not an expert on therapeutic equivalence. We can only go by what the FDA says, and their determinations are based off of the bioequivalence studies that the drug companies submit. If the studies show that the amount of active ingredient per 100mcg tab fall out of statistically acceptable range to be considered therapeuticall equivalent, who are we to say otherwise?
If there is variation between the different brands, then it would probably throw off prescribers or whoever is responsible for titrating the medications if non-equivalents were dispensed, because they'll never know what type the patients are getting (and even if they did, they wouldn't know how to act on that information). Even if the dose were being maintained the same, TSH levels can still fluctuate (studies have shown that the degree of TSH reductions vary depending on the brand) if each refill, patients were dispensed a different brand from before. If I bumped up a patient's dose from 100mcg to 125mcg, but the patient started getting a less potent brand, then I'd think it's not working, a higher dose is needed. Then I'll bump it up to 150mcg, and if a more potent brand get dispensed, all of a sudden I've over-titrated and the patient gets hyperthyroid, which can be pretty severe in the elderly.