You're right for the benefits, but there are also cons :
Each sub domain will be treated as a complete domain by SEs (more or less), so this means each sub domain will need backlinks, not only coming from you main domain.
More work while submitting to directories, even though you can image to submit more than one link at a time.
And, what you win from one part, the backlinks coming from your sub domains, you kind of lose it while linking back to them.
So the easy way is to go for folder structure, but if sud domains are active with good content, it can be good.
The principle is all sub domains should be worth a site, like a sub domain for a forum when running a big web site already, things like that.
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