They give you 850 renown when you sink 'em so well worth it!!!
I've seen several in Convoys near AL19 and AM52 (Rockall and Malin) and one down near Gib but always they've been American, and given that for me it's still 1940, it wouldn't be such a good idea to sink 'em!!!
In an old career I sank one flying the Merchant Navy ensign near New York in 1942 whilst on Paukenschlag patrol. Took 4 fish to bring her to a halt and then had to wait for the convoy to leave her before surfacing to finish the job with the 105mm.