I agree with both the PH and the TDS/PPM meter.(even in soil) I grow in soil, and I'm really glad I have a PPM meter to check things. It's nice to have that extra 'control'. It's nice to know what PPM you are feeding the plant, especially when you decide to check a sample of runoff. For example...if you're normal feed is 200 PPM and your runoff is coming out at 300 PPM, that shows that you have a little waste/fert buildup in the soil. If you have 800 PPM coming out, common sense would tell you that you have quite a bit of buildup. If you're feed is normally 600 PPM and comes out at 800, then common sense would tell you that it isn't a huge problem yet. Now, if you had no idea what PPM you were dumping into your soil, it would make the runoff alot harder to read, or better yet....'guess'.
I also find it handy for flushing, or knowing when to flush. Like I said above, if you are feeding 200 PPM, and the runoff is coming out at over 1,000 PPM, then your plant is going to thank you for having all that shit washed out of the soil. Also, if you have a problem and are suspecting 'nutrient lock', you can check the runoff to give you some more evidence to back that up. And, when i flush, I check the intitial runoff(just because i like to know.lol), then flush down to around 70 PPM, which is plenty low enough. Also, it saves time flushing. The main reason everybody says to use 3X the volume of their pot, in water, is because that'll make sure it's clean even if you have no idea how dirty it was to begin with. I very rarely have to use more than 2X the volume, and usually it's closer to 1 1/2, which saves alot of unnecessary time and effort. That alone, is worth the $20 for a TDS meter, if you ask me.