I understand most States may be beholden to those rules, but Texas is not. There are no requirements for security deposits to be in escrow or any other restrictions on their use, as long as they are accounted for and returned, when applicable, at the end of the lease.
TEX Prop. Code Sec. 92.101-109
and
http://www.nolo.com/legal-encyclopedia/texas-security-deposits-36234.html
and
http://texastenant.org/security_deposit.html
It seems to me in a "stacking nickels" business as I've seen it described, any small amount would help to improve a profit margin. I was just casually considering what a midsized property management business must have on hand in an security deposit account, and it is a huge sum of money. Security deposits add up quickly.
I realize it is other people's money, but so are bank deposits. But banks only keep a small fraction of the deposits on hand, and survive as a company by investing the rest (or at least that is how it used to work, once upon a time). Perhaps it is different because of FDIC insurance and so forth, but nevertheless, it is an industry that has made its living intelligently managing deposits in exactly the same manner.
If a property manager could conservatively improve their return by just a percentage or two, it could easily amount to thousands of dollars of revenue that could help improve a business margin.
Again - no idea what the laws are for other states or what proportion require specific accounting for the security deposit. This may just apply to Texans.