But if it's just a straight 19" wire, it will be a lousy 70cm antenna, even if it has a perfect SWR.
Well, "lousy" depends on what you expect from it. Not as good as one designed for dual band
use perhaps, but I recently used a straight 2m whip on 440 for a while and it hit the 440 repeaters
just fine. The antenna isn't optimum on 440, but due to the terrain I commute through the
440 repeater is either local or hidden behind some hills, and in that situation the 2m whip works
as well as anything else. In a flat area the dual band version will allow you to hit the repeater
from a longer distance.
So if your concern is simply hitting a local repeater, the straight 2m whip should be usable. In
marginal conditions you'll have better results with one designed for dual band use. But you won't
be able to tell the difference with an SWR meter, since both designs will have a relatively low
SWR on 440. The difference is in the vertical angle of radiation.