I can’t speak from much experience since I just finished my raft, but I’d say that it’s not necessary. If I was building a second raft, I’d probably leave it off for now and decide later.
That said, I did mount mine already and I extended it with some silicone hose so it can be used from the normal seating position. But if you aren’t going to extend it like that, then you’re going to have to twist your body around to get to it (seeing how most people mount them beside the boston valve on the back of the raft)… and in that scenario you might as well just use the boston valve.
The boston valve has a one way valve in it so it won’t let air out between breaths, whereas the topup valve doesn’t have this feature.