He is suggesting it because belt works "all or nothing":
You either have enough to bounce or not, and, if you only armor up till you lose speed, then you won't be able to bounce anything.
On the other hand, bulge, when hit by a torpedo gets the damage until it is reduced to zero, when the hull starts taking damage too. So,an extra 2-3 bulge would reduce the amount a torp (the first one only) does, but 0.2-0.3 extra belt would do nothing against enemy shells...
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