So, since the piano is probably out of tune - we shouldn't invest in a good tuner?
And since the piano is probably out of tune, we have to tune to it and say... for a couple of notes, we have to play them accordingly on the guitar?
When playing in a band, one is said correctly tuned, relative to the rest of the band. Even if they are out of tune to the rest of the world, they will sound good. In this situation, the best tuner you have, will make you in tune, relative to the "correct frequency", but you will end up out of tune to the rest of the band (remember, we use a grand piano in this topic. You can't just retune an grand piano to your guitar).
If you play in a band that does not have dificulty to retune to your instrument (e.g. only bass, and guitar), yes, you can use the tuner as a guideline, so please buy it.
If you play in 1-man band, yes, you probably need it, so please buy it.
If you play strictly in bed room, yes, please buy it. You need to know what's the correct tune anyway
So, as I said earlier, to each, his own. If you think you need it, buy it. But doesn't mean everyone does. Besides, music is not a rocket science, don't make it too complicated and restricted. "Slightly out of tune" doesn't always mean bad in, pun intended
