Leaving the bottom half of hair alone from bleach lead to healthier hair?

So you mean doing just the roots?  Of course it'll be healthier. Bleaching over the same hair time and time again is adding more and more damage that it can never fully recover from, in a sense it never recovers as hair is dead as soon as it leaves the scalp. All you can do is patch up the damage, but you can push it too far.

I am not sure i understand your question. Do you mean leave the underlayer natural and just keep the roots blonde on the top layer? Then bleaching after 3 months? Waiting doesn't make bleach less damaging, you should ideally only bleach your hair once, whether you have 6 months of regrowth or 6 weeks.
If you have layers you could leave the underlayer natural or just highlight, and just keep bleaching the top layers (which would be the shortest part of your hair) This is a good way to keep length and condition, because only the most visible parts of the hair get bleached, and the longest (oldest and most fragile) hair gets processed less.

I was confused too.

The only thing I'd disagree with a little there Wicked Pixie, is that the top layers of hair are very often most damaged and fragile just from day to day wind, sun, pillow friction etc. so while shorter, can take less.

To me, it sounds like you're saying hair grows from the bottom, instead of the root. Is that what you meant? 🙂

True, my topmost layer is always more frizzy due to sun exposure etc. It is a good way to cheat your way to bleached hair that i have used in the past though, You only really need to bleach a small portion of your hair for it to look like that colour if you have long hair. I have been willing to sacrifice length on the top layer but kept the rest much longer and less processed, so my hair was still the same length just more layered. It's always a compromise between length/colour/condition when bleaching long hair.

What I mean is leaving all the hair like from the mid ear down untouched  it's like when you get a partial highlight and they only put foils on the top 2/3 of your head. That bottom layer of hair is not colored at all.