Moisture Meter YES or NO? (Could harm the roots?)

Nope, it won't hurt your plant. And who knows, it might be useful to you. Different strokes for different folks and all that jazz. I say test it out and see if you like it. Good luck!

LOL! I bought one of those with TWO prongs and I felt the same way you do! But they really DON'T hurt the roots so don't worry about it. I never measured moisture with it although I could... I found (and still find) that picking up the pot is a good indicator of whether or not a plant needs something... pick it up after you've given it enough water so that you've had some runoff - you'll feel how heavy it can be... then pick it up when the top looks dry/crumbly and you'll see how lightweight it is... it gets to the point where you can actually identify areas of the pot that are wetter than other parts! One side might feel heavier than the other... Anyway, when the pot feels super light, you need to water/feed... if you can still feel good weight in it, leave it alone. Good luck!

Otherwise, for the device itself, it's better to plant it, take the measurement, then take it out, clean it well (anode and cathode), and when you need it again, you plant it again.

Not a fan of this crap style (oops, sorry) Seriously, water the soil until the water flows slightly down (grow bags are ideal, it prevents root rot and it's cheaper than this thing in the photo... and once watered, you wait for the top 2cm of soil to dry before watering again, it's not more complicated ^^