DIYbanter - 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