As can be seen, for a single Major Flaw, Unstructured Caster is huge - it's the kind of Flaw that you only take because you want to set a limitation and work around it (or because you're a perfect example of the species rattus laboratorium). Furthermore, due to the double-hit on UC, most people who take Unstructured Caster take Rigid (and often a couple of attached flaws like Loose Magic because you might as well munch out if you're doing this) and just forget about ever learning a spell. Third, for pre-Schism games or the inevitable special snowflake who wants to play a Diedne survivor, Covenants 91 mentions that the druids didn't have Formulaic magic, but they did know how to cast great rituals - so a more "old-school" Diedne practitioner might want to have something like this. And fourth, Rigid Caster does also lock out the minor option of spending vis on spontaneous magic.
Again, if you're a master enchanter Verditius who doesn't give a whomp about ever casting, all this is fine. But I'd like to give just a little more flexibility to the Flaws in question (whether for an "old-school druid" Diedne in a pre-Schism game, a Verditius with LLSM who actually does cast some spells every now and then, or a really bloody weird Mercurian), and provide a reason why you might not take both. (While I'm reducing the strength of the flaws a bit, they still qualify for Major in my opinion - they're both a bit bigger than Weak Spontaneous Magic.)
Rigid Magic (Minor Hermetic Flaw)
You cannot use vis to enhance spell rolls, in certamen or for vis boosting. You may cast ritual magic normally and use vis in the laboratory.
Small Magic (Major Hermetic Flaw)
You cannot cast ritual spells, limiting the breadth and strength of the effects you can use and preventing you from using magic to make permanent changes. You may use spontaneous and formulaic spells normally.
Unstructured Caster (Major Hermetic Flaw)
Whether because of a flaw in your Gift or the training of your parens, you have never developed the ability to cast formulaic spells. Ritual spells are unaffected. If you are a member of House Verditius (where this Virtue is slightly more common than elsewhere), or with troupe discretion otherwise, you may take 300 XP (plus 75 XP for Skilled Parens, minus 75 XP for Weak Parens) during apprenticeship and learn no spells. Outside of House Verditius, consider taking the Hedge Wizard Flaw.