A properly calibrated device can measure something meaningful, but it's still not all that useful as the bandwidth is unspecified. If you're measuring everything, then you don't know what frequency it was from... it could be anything... noise, a signal, a nearby radio station, who knows. That can be tricky, for RF purposes.
The simplest meters are also peak reading, which means one signal dominates, and if it's rather peaky, it comes through much more strongly than the rest. Which might be what you want -- say, to simulate the response of a radio receiver with AGC (which takes some time to respond to peak levels, but follows them nicely).
Offhand, most airborne signals are in the mV/m range or less, which means, for a roughly resonant length antenna, you get about as many mV at the terminals. Distant signals go into the uV, or under the noise floor. High gain (very directional) antennas will get more, but not hugely more (The biggest, Arecibo, is 73.5dB, which is only ~three orders of magnitude -- okay, to be fair, it is pretty huge gain... but it's a pretty huge antenna, too!).
You can observe signals of this level with merely a scope (most can resolve a few mV), but you might not be able to get a good trigger on it (peaky signals dominate the trigger, and stable displays only come from periodic waveforms -- that is, anything with a strong spectral peak).
EMC measurements are rather more specific; they're done with a specific detector, bandwidth and sweep rate. The fixed bandwidth means you'll get a very different number from broad signals, and converting between the two measurements isn't simple (you have to RMS over the entire spectrum to find what a wideband voltmeter would measure).
Now, in combination with a tunable filter, you basically have a spectrum analyzer -- so, while the voltmeter may be kind of dubious by itself, it's the heart of quite a lot of powerful devices! (And how would you make a tunable filter? You use one very good filter, and hetrodyne the input signal around it, of course!)
Tim