Client like thunderbird is good if you always use the same desktop/laptop machine to do your email. If you are using multiple devices like school, friend, work, library or even mobile it totally breaks down. To say nothing of system failures, breaking or losing the machine etc.
Most people who love TB have a setup that has been stable for 20 years. Good for them, it suits their needs. But the contempt with which they seem to hold the majority of the population for whom TB would be a totally unsuitable choice is rather unpleasent.
Ever notice how rarely you see someone saying “I switched to TB from webmail 2 years ago and its great”?
Too bad, as i would absolutely love to switch the floss desktop/mobile clients and have tried to do so on a few occasions. They are simply not compatible with modern communications habits.