Curiously, according to the Google Ngram Viewer, the usage of "motive", "motives" has been steadily declining since 1800.
In contrast, usage of "motivate", "motivates", and "motivated" has risen to overtake the former words.
Could this be because "academics" favored a more latin sounding word?

"During the 17th and 18th centuries, dictionary writers and grammarians generally felt
that English was an imperfect language whereas Latin was perfect. In order to improve
the language, they deliberately made up a lot of English words from Latin words. For
example, fraternity, from Latin fraternitas, was thought to be better than the native
English word brotherhood. "
is the above true?

@WW You might like to look at the band descriptors for academic IELTS writing:
level 8 :"skilfully uses uncommon lexical items.......
level 7 :"uses less common lexical items with some awareness of style and collocation...
To me these benchmarks raise noteworthy questions about how common is "less common" and exactly what corpus one would use to come up with an answer. Of course in truth I use the corpus in my head, however "standard" or unaware of US/India usage that may be.

@WW "Handful" was somewhat under-stated I grant. I think I'll leave it up to you to come up with how many Brits indeed speak "proper" English (taking that as Standard Brit English) and use "proper" pronunciation - whatever the limits of that is - should one weed out all those Scots and Northerners?
If we are grounding our thinking about what is "mainstream" English on numbers then English as spoken and used in India surely is worthy of a silver not a bronze..

Ah yes I am thinking it is about time someone was asking those hundred million plus English-speakers in India to be making their input heard, instead of us relying on a handful of Brits with their "proper" grammar and pronunciation.

@WW Phrasal verbs is the one area where I think translation helps with a monolingual class. Might I suggest "turning the tables" - getting the students to teach you the Polish for a few verbs like "come up with", and then test you a week later?

@WW Yes language learning in continental Europe is much better than in England. As I haven't betrod the hallowed cloisters of an English school in many a year, I have no real idea what goes on; you are clearly au courant. My one-time school still includes a modern language in the core curriculum, but Latin and Greek are optional extras (but sadly not Russian any more).
I think one's views are modified by one's experiences in life; and when I started TEFL-ing years ago i would have wholeheartedly agreed with you all the way down the line. However, over the last decade I have been teaching quite a bit of academic English - the sort of stuff where one awards no marks for "the weather is dry" but a tick for "the climate is arid". It's qiote different to CPE where one is aiming for a range of more normal English.
With your knowledge of Spanish/French I am sure you will appreciate that academic English is, in terms of vocabulary, actually easier than phrasal verbs for Romance language speakers.
When it comes to non-Indo-European language speakers though, the boot is on the other foot - so much so that on some occasions I ended up spending a couple of hours, inter aleae, teaching pure latin in the midst of an EAP course. Surpriisingly this went down well as it brought some cohesion and structure to learning those 2000 non-technical academic words that are sine qua non. But after a while one must wonder at the unhingedness of it all, teaching Latin to Chinese so they can learn English. Would help if they learnt Greek too, I suppose.
However I don't bother with latin plurals much - it's all about word-roots, pre-fixes and suffixes.
"Hi Teng Shu-ping, what's the weather like in Xin Jiang?"
"The climate is arid, sir".
Pax tecum.

@WW The downside to learning Latin is that it is just reading. Scour the net for listening if you will. It's also hard to do any writing with so many modern words like airport and coffee just missing. Speaking practice? No,not even at the Vatican.
Put this all alongside learning Spanish (think South America) or another Romance tongue, and the difference is clear. Latin is often touted as being useful to understanding English; it is. But any other romance language wil do much the same.
Sometimes word-roots are a good "mind-hook" or aide-memoire when learning a "foreign" tongue. So may/might in English becomes "moc" (can) in Polish, and "mighty" becomes "mocny". Of course this won't work if one is learning Chinese or Turkish.....
For me (and people are different) I find the key to speaking a language is in being widely read with a huge word-stock, and listening at least three or four hours a day. With the internet one can watch the news/TV in almost any tongue and copy stuff to the iPod and listen to it as one drives along. Can't do that with Latin.
Lastly, if education is in part about broadening one's horizons then a truly "foreign" (ie non-Indo-european) language would be better.

Re gnosis: perhaps I should also have mentioned agnostic,cognizance,reconnaisance, recognize, and in the end "acknowledge" as having like roots.
Re crisis: also related to discriminate, discern, and "riddled with" and "riddle" in the meaning of sieve.

@Brus Quite right, but they were Greek nouns before they came into latin.
'Krisis' is somehow related to kriterion and kritic from a verb meaning to deem, judge.
'Thesis' is somehow related to our word 'deed'; and 'para' means alongside as in parallel, paramedic.
'Diagnosis' = "through-knowing" - dia = through; gnosis as in gnostic is our word know,
or in slavic tongues znac/znat. In latin it became nosco,noscere,notus or -gnosco as in cognosco giving us cognoscenti and I guess cognitive, and ignore.
'Oasis' is a greek borrowing from tongue in west asia where they truly had deserts.
"iris" actually was borrowed into latin from greek and means a rainbow - so an iris is a "rainbow-bloom" if you will. The plural in latin is "irides" .