I am currently looking to send my Statement of Purpose to an English TA for editing. The problem is that I am afraid my Statement of Purpose might get plagiarized. However, since my statement is highly personalized, is my worry unnecessary?
This is very unlikely if you do your statement of purpose well: the document should be too specific to your situation and interests to apply to your English TA.
To reduce the chances of it being plagiarised, try considering some standard tips for a good statement of purpose [emphasis mine]:
Tell them what you’re interested in, and perhaps, what sparked your desire for graduate study. This should be short and to the point; don’t spend a great deal of time on autobiography.
[Summarize...] [r]esearch you conducted. Indicate with whom, the title of the project, what your responsibilities were, and the outcome. Write technically, or in the style of your discipline. Professors are the people who read these statements.
And so on. The common theme among these and the remaining tips is that the statement of purpose should really highlight your unique qualifications and interests.
If you and the TA are careful with your email accounts, and change passwords regularly, etc., you can trust email with your document. But you need to be trained in using a spell checker, and you should work with your TA in person (over at least two appointments) on improving your essay. Otherwise, you'd be using the TA as a glorified copy editor. That's not fair to him or her, and it doesn't help you produce an admission-winning essay.
You assisted a grad student in robotics research as an undergrad. This would imply you're probably not a liberal arts student. The edit history of your posts by others also points to you not being a native English speaker. There is actually nothing wrong with that. I am also not a native English speaker. I am just pointing out what should already be obvious to you about your situation.
On the other hand since you're in Canada, this would mean that your English TA is probably a native English speaker and most likely not in the same field of study as yours.
Not only such a person would have no use for your statement of purpose in his field of study, but you must be overestimating the quality of your writing in English, if you believe that it could be useful for him to give to any of his other ESL students (who may be applying for grad school in the same field of study as yours). It simply wouldn't be.