I agree it's a great strategy. That said, there are some problems that can arise.
1. The property has to appraise for that higher value. Especially in markets that are still growing and competitive, this may not be an option. If you ask for it up front and the property DOESN'T appraise for the contract value, then the seller's first response is to lower the price but take away those closing costs. And if you're counting on the appraisal to be higher, and it doesn't, are you now in trouble? Don't count on it, but definitely worth a try!
2. There are limits to what a seller can contribute to your purchase. For example, they can't pay your down payment. It can only be used for closing costs. Also, it depends on how much you put in a down payment as to how much you can get in contributions. If you're buying an investment property, 2% of the purchase price is max, no matter what. Owner occupied loans, <5% downpayment, 3% is the max. 5-10%, 6% is the max, etc. Definitely check with your lender that they can use whatever you're negotiating so you aren't left with too much money and no where to put it.
3. Asking for it up front can make repair requests challenging (at least in NC and depending on the situation). Most sellers want to give a credit at closing rather than repair the home. If you already have closing costs negotiated up front, you may not be able to that, and then the seller simply doesn't do anything.
On the pro side, you can use those seller closing costs contributions to do interest rate buy down programs as well (owner occupied only I believe). You can only use seller funds for those 3-2-1, 2-1, 1-0 buy down programs.
At the end of the day, it is definitely a great strategy to keep more money in your pocket and/or save you money in the first few years on interest. Make sure you talk to your Realtor about the pros/cons and how to best move forward!