I mean, I think that's basically correct, though even if Roman Quinn hit .400 between June 15 (which is probably optimistic) and July 15 I wouldn't be making any decisions based on him being on a September line-up card.
Beyond that, it's an arbitrage play. If Keuchel or Kimbrel are available at a price you like, for which you also get two months of production and don't have to give anything up, that has considerable value, and the area of need is pretty clear. Yes, they are in first place but also don't look likely to separate themselves from the Braves or be an NLCS favorite.
Wait-and-see also has value - you don't spend money, and if someone unexpected hits or an injured guy comes back strong, you haven't spent anything at all. If those things don't happen, you just end up paying more to fix it.
I think the Phillies will probably pay the price to get guys they like at the deadline, and they'll either be controllable or reasonably cheap if not. Of course there are also salary dump type situations. You sign Keuchel now you've got no chance of getting Greinke. Not saying they should, figuring out that contract would be rough, but, optionality.