Depends on where you live.
I put a DAB radio in my old Passat, and had one in my old BMW. I liked the extra stations I could pick up, but unfortunately, coverage in my area isn't great. So it would often drop out on some of the stations I listened to. That said, there are some parts of the network around here where FM drops out too.
None the less, I wish I'd went for the 315 DAB rather than the RNS510 I ended up getting. If it were me, I'd definitely install the aerial. Not just for the "tick in the box", but because all your missing is the aerial, and why miss out on functionality you already have for the sake of a £20 aerial and 20 mins running it.