Studies show that merely using a spelling curriculum has no effect on student writing. None.
There are authors who claim that THEIR curriculum DOES have an effect on student writing, especially with LD students. To be honest, I use spelling curricula more to facilitate teaching cursive handwriting, to teach reading, and to satisfy my OCD needs, rather than to teach spelling itself. Romalda Spalding says students cannot write in cursive until they have progressed to writing in syllables, from individual letters/phonograms. I've found this to be true. I teach an intensive mix of cursive handwriting, spelling, dictionary respellings and some basic grammar, using a lot of copywork and sentence compositions.
I found the section on spelling in Homeschooling on the Cheap to be interesting reading. The Kindle version is $4.99
Spelling When it comes to spelling, there are three methods that are both effective and inexpensive. You can teach using dictation, using your children’s own misspelled words, or by using word families. All have their merits, and are highly superior to traditional spelling programs.
Traditional programs tend to follow one of two routes, or a combination of both. They either teach using phonetic rules – all the words in this week’s list contain some from of long E, or exemplify the two sounds of G, for example. These programs work great if your child is capable of reading all the words on each week’s list. Or they teach the spelling “rules” – all of this week’s words have an I before E, or are all plurals using –ES. These programs work wonderfully if your child is able to transfer what he’s learned in spelling to other words with similar pronunciations. If your child isn’t a strong enough reader, or has difficulty relating spelling rules to writing on his own, then all the wonderful programs in the world won’t improve his spelling abilities. Stewart, Suzanne (2010-07-15). Homeschooling on the Cheap (p. 42). . Kindle Edition.
The author then goes on to explain how she does dictation, misspellings, and word families, and then says this:
Never, regardless of the method you choose for teaching spelling, allow a spelling error to go unchecked and uncorrected. The purpose of teaching spelling is so that children learn to spell correctly. Allowing misspelled words to remain misspelled runs risks of the student mis-learning the word with the incorrect spelling. Then, the tougher job of relearning the word correctly must take place at some later time. All spelling errors in dictations, both initial and final, all spelling errors in the week’s copywork or practice exercises, should be corrected immediately by the student for correct spellings to be cemented in the student’s mind. I don’t recommend really starting spelling instruction with children before 8 years of age or so. Their language abilities should be focused on learning to read, write and enjoyably experience language – phonics, handwriting, composition and creative writing, literature (read alouds,) and memorization – and not worrying them with the nitty-gritty stuff like proper spelling. They’ll not be doing much assigned independent writing at this age, anyway, so spelling instruction is a bit unnecessary in my book. Stewart, Suzanne (2010-07-15). Homeschooling on the Cheap (pp. 47-48). . Kindle Edition.
I don't think it's wrong not to use a spelling curriculum. All too often they are a waste of time. Yeah, I use one, and HOPE it's actually worth it, but I'm not so sure it is. The students ARE learning handwriting and grammar during spelling though, soooo...I keep teaching it.