thumblatch lock set vs. separate locks?

I've got a four-year-old imitation antique thumblatch lock set from Restoration Hardware. It's a single unit, with a keyed deadbolt at the top and a thumblatch for opening the simple latch below the deadbolt. The latch itself does not lock; only the deadbolt locks.
Yesterday the deadbolt, which had been sticking a bit every so often, suddenly jammed in the locked position. Neither the key nor the interior latchknob would open the deadbolt.
I called a locksmith who came out, pried out the deadbolt itself (after determining that no amount of tweaking would unjam it), and then suggested that (a.) replacing the entire unit with another thumblatch set would cost way more than replacing the deadbolt and lower latch as separate (keyed) items, and that (b.) thumblatch set units were inherently more likely to break than separate units.
In a hurry and needing something to lock my entry door, I had him put in a new deadbolt and a new keyed latch in place of the unit piece I'd had. They work fine. Of course now I've got to patch and refinish and repaint my entry door, since the old unit left its imprint on it pretty severely.
But I got to thinking: the deadbolt in a unit thumblatch set is not MECHANICALLY connected to the latch portion. They are simply two items combined in a framework. So why would unit sets like this be more liable to breakage/wear than completely independent pieces?
Yes, there's more security in having a deadbolt AND a keyed latch on a door than merely having a deadbolt and a thumblatch. But that's a separate issue. Is there any mechanical reason for choosing separate deadbolt and latch over a unit?
Jim Beaver
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Surprised at no response to this at all. Anyone have an idea about this?
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