It depends on what the Romulans were after. In this case I figured they wanted to replace a Starfleet captain as a long term spy. There's no reason to assume they'd always want some top level brass, in fact, there probably would be greater chances of detection since higher and more well known Starfleet officers would be better known and any replacement easier to spot. But assuming Picard was just some average captain at that time, there was probably less likelihood of accidently being uncovered, and he could still be incredibly useful. High enough on the food chain to do real damage and gain intel, but low enough that he's not as likely to be exposed.