Why not? Maybe T'Rul was suspect politically, and they sent her with an obsolete cloak to test her loyalty. Maybe they were even afraid anyone they sent to a babysit the cloak would defect, so sent someone unimportant with old equipment.
Anyway, I don't see why the rank of "sub-commander" has to mean she's a person of any importance in the military. It could've been a bonus given as an incentive for her to go play with the enemy.
I see what you mean. But what I keep coming back to is her behavior throughout the episode. Even an obsolete cloak with have held some value for the Federation, as they had no cloaking technology beyond what Pressman illegally developed on the Pegasus
. T'Rul doesn't allow any Defiant
crew members near it, yet she willingly divulges secrets that the Romulans have never shared with Starfleet, something I wouldn't think a person with only limited authority would be allowed to do.
OTOH, it's possible that she was sent not only to watch the cloak but also to gather her own intelligence about the Dominion. As far as we know, she was the first Romulan to encounter any Dominion member species. It makes the most sense for the Romulans to send someone who would be capable of observing their new enemy for weakness and reporting back her findings to her people. She may have gotten a bump in rank as an enticement, but I still don't think she'd have been used unless her superiors found her useful.