To me, Jon Daniels' decision to trade for Ryan Dempster at the last minute was very much an impulse move, which rarely works out (see: the Yankees' signing of Kei Igawa in response to the Red Sox's signing of Daisuke Matsuzaka, as one example).
First, the Rangers' biggest rival, the Angels, acquired yet another ace in Zack Greinke over the weekend. They now trot out a rotation of Jered Weaver, C.J. Wilson, Greinke, and perhaps Dan Haren and Ervin Santana (depending on health and performance—formidable both for the regular season and playoff series.
Then, other potential front-end starters fell off the market. Cole Hamels had already re-signed with the Phillies, the Marlins decided to hold on to Josh Johnson, the Red Sox kept Josh Beckett, and the Cubs were asking too high a price for Matt Garza.
Around 2:00 this afternoon, it started being reported that the Yankees were in on Ryan Dempster, too. If they acquired him, yet another team the Rangers may play in the playoffs would have obtained starting pitching help while the Rangers acquired none. (In the last couple of weeks, the Tigers got Anibal Sanchez, and the White Sox traded for Francisco Liriano.)
The Rangers also just found out that starter Neftali Feliz would need Tommy John surgery and had to process that information in a fairly short time.
I think Jon Daniels panicked, decided he needed to make a deal and traded for Ryan Dempster.
Don't get me wrong. I think this was a trade worth making, particularly due to the low cost of acquiring Dempster. But I don't think he is the ace that the Rangers needed.