If it was necessary the system wouldn't run without it. It might be nice to have. But so many voltmeters, especially cheap ones, are inaccurate to the point they're nearly useless. The difference between the battery's nominal voltage and the charging voltage might only be a volt or volt and a half. If the charging voltage is supposed to be 13.8 volts, and is actually doing 13.8 volts, but the cheap voltmeter shows 13.5 or 14.3 volts, the pilot might get all worked up and worried, all for nothing. There is such a thing as too much information, and we see it sometimes once a guy gets a multi-point EGT/CHT system in his airplane and finds significant temp spreads between cylinders and gets all sweaty over them. They were there all along and are a feature of primitive induction and exhaust and cooling systems and you might spend big bucks trying to get the stupid numbers to line up.
Voltage drop during cranking is affected by much more than the battery. Many perfectly good batteries are replaced when the starter gets lazy. Or the starter gets replaced. And gets replaced again because the new one is lazy, too. The real problem is often old and burned or oxidized contacts in the master or starter contactors, those little things that cost $50 or whatever and take less time to replace than it does to get the cowl off and on again. At 250 cranking amps, a tiny resistance can cause a huge voltage drop. Volts equals amps times ohms. If the drop is across the starter contactor, it will not show up on your voltmeter, which is connected to the bus. If it's the master contactor it will, but it doesn't tell you whether it's the battery or contactor or a corroded cable terminal that's doing it. It's just information without useful direction.
