It implies he does not have sexy curves to lose like Ricchan.

Seishin is stated to change into western clothes before going to Yamairi and before visiting Toshio once, so I assume in the novel he's not in his robes all day. I suspect that him wearing the same work outfit constantly is a matter of visual convenience for we the viewers, the same way Ozaki wears his white coat in every single scene he is in that is not a funeral or him in bed. Considering the clothing Fujiryuu tended to draw on characters in Shiki when they're not in a work or school uniform, I think the Waka-Combi are lucky to have job uniforms, really...