New strings go flat many times faster than old ones. Old strings, when broken in, can hold a desent tuing for months. The sound of strings deteriorates due to build up of dirt on strings. Boiling strings in water will revitilizes sound. Vibrato does not induce intonation problems.
Intonation is depentant on control of one's voice and vocal muscle strength.
I believe vibrato should be learned to provide artistic contrast and added depth to song much the way black is used to create depth in paintings. An all-black painting would not be very interesting. Neither would an all-white one. James I found your article interesting but a bit off the beaten path. I imagined in my mind's ear what it would be like to hear Puccinian phrase sung in a vibrato-less manner.