It's an arms race situation. Once you give me an information channel like a "word document", I've got an endless variety of ways to encode other things into it. I can encode bits as English sentences or other things that will be arbitrarily hard to pick up by scanning.
If I were Google, I wouldn't try to pick up on the content, I'd be looking for characteristic access patterns. It's harder to catch uploads, since "new account uploads lots of potentially large documents" isn't something you can immediately block, but "oh, look, here's several large files that are always accessed in consecutive order very quickly" would be harder to hide. It's still an arms race after that (e.g., "but what if I access them really slowly?"), but while Google would find a hard time conclusively winning this race in the technical sense, they can win enough that this isn't fun or a cost-effectively technique anymore (e.g. "then you're getting your files really slowly, so where's the fun in that?"), which is close enough to victory for them.
So, I'd say, enjoy it while you can. If it gets big enough to annoy, it'll get eliminated.
They can just throttle access to Google documents to something like 4 GB per hour and then block obvious abuses. If people start encoding bits as English sentences they are reducing the amount of useful data they can download within an hour which is exactly what you want.
"They can just throttle access to Google documents to something like 4 GB per hour"
No, that's not likely to work. I'm sure there's far more legitimate users using 4GB of documents per hour than abusers right now. You have to remember things like bulk downloading, bulk scanning, bulk backing-up, shared automated accounts doing all sorts of legit things, etc. are all legitimate use cases. You can't just throw out all "big uses" or your enterprise customers are going to pitch a fit, and that's a bigger problem than people abusing your storage for a while.
(Those things will still have different access patterns than abusers, but thinking about how that will manifest and could be detected is an good exercise for the reader.)
I'd say most certainly do not try this. Do you want to loose access to your gmail, maps, contacts, whatever else you rely on Google for, because you were found abusing google drive?
Seems like more of a "can" rather than anyone actually using it. Always interesting to see how something can be broken or exploited, even though it may not be practical
If this considered an abuse-of-services now, the terms could be updated to clarify.
The finger print of big chunks of base-64 encoded blobs in Google Docs could be easy to spot.
If Google cares to notice this and take action, they can and will.