Thinking out loud - A fairly large multi family with a standby generator for an elevator and some common area loads. Since we have it, we may as well put the exit lighting on the generator. Bt that means a separate transfer switch and distribution for the life safety loads (exit lighting). Do you think the elimination of a few dozen emergency battey pack lights is worth the added cost of the transfer switch, distribution, and associated generator up-grades?

It depends if the upgrade costs are less than the ongoing costs of yearly battery light inspections (required by law here) and the eventual replacement of the battery packs when they go bad.

If the power goes out it's worth every penny. If it doesn't ...

What is the history of power outages, there?

In my experience any attempt to NOT use the backup gen-set 'blew up.'

BTW, there's no 'extra' switch required, at least not in my systems.

The critical loads are normally run through a sub-panel -- which is switched, entire, when Poco power drops out -- with, say, a seven second delay.

You will very likely need a separate ATS and panel depending on how your AHJ enforces the Code.
Emergency lighting and exit signage comes under Article 700 - Emergency Systems whereas elevators, security cameras and the like generally come under Article 702 - Optional Standby Loads.

The majority of the AHJs that I know will not allow you to mix those loads on a single ATS or a single panel.

Gentlemen:
I suggest that the OP start by reading the definitions within 700.2; 701.2; & 702.2 to clarify what may be the correct steps in addressing this subject.

Based on what the OP ststed, IMHO, the existing gen is not a 'Life Safety' power source, as again IMHO, the egress lighting is 'Life Safety' in a multi-family structure.

Also, keep in mind that different localities have different opinions and codes regarding life safety issues. The best place to start is your local AHJ, either Electrical, Building or Fire.

The space would required quite a few emergency (battery) lights. Normally I would eliminate the battery emergency lights and connect some of the building light fixtures to "emergency circuits". In this case however the distances of the emergency feeders would be significant. (100A feeder) about 200 ft from the generator to the transfer switch and another 400 ft to the other side of the building.
So instead of the emergency lights we would have:
600 feet feeder.
Two 100A panelboards.
100A transfer switch.
What ever the additional cost of the generator to make it emergency (instaed of just standby).

Does this convert the unit definition from a standard standby unit to life a safety unit.
A life and safety unit here requires much more testing, annual certification and logging of maintenance records then a standard standby unit.