Air Carrier descending into BWI reported a TCA TA with ATC reported traffic observed on TCAS; the reporter questioning ATC's separation judgment.

Narrative

Upon executing an ATC descent to the RUANE fix on the EMI 5 Arrival; we were informed that there was a Cessna Citation at 12;000'. Looking at the TCAS; I saw a plot ahead of us traveling right along the magenta line where our flight path was. As we were descending from 15;000'; our previous restriction; to our new restriction at RUANE of 11;000'; I thought it was odd that the airspace below us was not clear. Usually if there is a conflict you will normally hear something like 'maintain a minimum of 1500 FPM in the descent;' or something like that. In this case; I manually increased our descent rate to 1800 FPM; and as the flight got closer; we received one 'TA' caution. Then; we were clear. I was half expecting a 'RA' and to disengage the autopilot and take evasive action; but it the increased descent rate turned out to be sufficient. I'm writing this report because it just seems odd to me that had things been left in their previous state we would have been on a head on collision course with the Citation. I normally don't see that in a positive controlled environment. Normally there is some lateral separation or some specific instructions given by ATC to achieve separation. Maybe this could be looked into. Did I miss something? Is this normal? I'm a little worried that had we been busy my attention would not have been focused on the Citation and we would have been one failure away (TCAS) from a midair. Maybe it's just me; but it struck me as a little odd.