While it's easy to rationalize away a word limit, it's actually a strong indication of whether you can adhere to the admissions requirements. They have word limits for a reason:
1) comparability -- if they're deciding between applicants and they put your reco side by side with someone else's, it's easier if they're within the same word limit
2) focus -- they want to know that you can prioritize elements of your background as the most critical and that your recommenders will back you up on those
Technical issues with overrunning:
-- Everything is in the cloud now -- the display screen may only show the first 300 words and cut off the rest -- not so good if your recommender had put in a fantastic finishing endorsement line.
-- May not display properly - you don't want anything to interfere with the actually consumption of a strong recommendation.
You get where I'm going with this. Stay within the word limits. Demonstrate you can deliver to specifications.