It's all about communicating with readers, as far as I am concerned. We write novels for that purpose (well, I do ... mostly).
I want to write a fantasy novel using my Medieval background. Goodness knows I have enough Middle Ages. What keeps holding me back is that I don't read the Middle Ages in translation. When I read a chanson de geste or a roman, I read hip language and dialect and words that have a limited currency, in Old French or Middle English generally. Hip stuff: but not modern stuff. In fact, fashion is one of the things I read for, as a cultural historian. So when I want to write that medieval fantasy, I get the urge to write it with the modern equivalent of my fabbo Old French vocabulary.
Say I want to use 'grok' to denote Julian of Norwich communing with her deity. I know if I do that, I will get a violent outcry, because underlying most of the views about how we should read the Middle Ages is that somehow they were never hip. And besides, grok has limited modern currency. It would express my views of Julian's numinous perfectly, in one simple four letter word, but it would go directly against reader assumptions. And reader assumptions count. At the very least readers will have to stop and ask "Why did Gillian do that?" Instead of bringing them closer to the novel it will distance them.
What is really a shame is that this also applies to a lot of genuine Medieval constructs. We as readers bring a bunch of perceptions and interpretations to every single word we read. I was talking this over with an historical fiction author yesterday. We were looking at a whole series of words where meaning has shifted and which needed considering carefully if she wanted to use them in a novel. Some had technical meanings in the Middle Ages which had to be matched very carefully to how they were used in the text ('manumission' for instance), while some bring up thoughts of the Middle Ages that reflect nineteenth century romanticism in minds of most readers. 'Forest law' mostly brings up Marryat's views of people starving and dispossessed, for instance. Some got dumped. Some got used, but with care. It was a very interesting conversation.
One thing that my little scholarly project has brought home (um, maybe 'little' is not the right word) is that every single writer wrestles with this problem. Some choose to create the feeling of archaic language through invention and reconstruction. Some choose to seek more neutral language. Some move towards sentence lengths and the shape and feel of the text to create a feel for the period, while some seek specific vocabulary that will precisely illuminate. There is no right or wrong.
What it all comes down to is what we enjoy as readers. In the final analysis, whether you want your historical or fantasy novel to have 'tis' and 'twas' or 'grok' or something else entirely pushes you to read certain authors, and then you, as reader, congregate with people who enjoy the same books as you. And then we all get impressed at how right we are in choosing language that so cleverly reflects the Middle Ages while fitting modern sensibilities, not realising that there are another thirty groups out there, doing the same but with a slightly different sense of what language cleverly reflects the past.
There are so many different groups of people who love the Middle Ages and they all use different techniques for making it tangible. Groups like Regia Anglorum literally make the Middle Ages tangible through research and re-enactment. When they read, I suspect that grokking and twas might be secondary to the feel of the armour and the heft of the sword. Lovers of pure romance fiction are more likely (though definitely not always likely) to prefer some intentional archaisms - it is part of the feel of the Romantic Middle Ages. And Medievalists are going to get a huge kick out of finding holes in everything. (Me, I put errors in on purpose, just to annoy my colleagues. How to win friends and influence people.)
The bottom line is that writers need to write good books, and know their readers. I doubt I could get away with grokking, alas, because it mixes genre. It is a science-fiction word and will sure as sure lift readers out of a novel set in the Middle Ages. I could use it as a signal I am creating a science-fictional reality, but not as a signal I am accurately depicting the life of a rather interesting English mystic. So it is not a matter of right or wrong; it is a matter of what you want to signal and to whom.
The language in a novel reflects the crossroads where the shape of the story, the shape of the perceived past, and the reader's wishes all meet. Reader's wishes change. Generally speaking, readers of historical fiction tend to prefer a higher level of historical accuracy now than a few years ago, for instance. I love this because I can walk into a convention or a bookshop and end up involved in long satisfying conversations about history. I hate this because I still can't use 'grok' in a novel about Julian of Norwich.