I like Battle High.
I like that it gives a game a different dimension. Not only you need to see the score, but also you need to remember which team is the strongest, and act accordingly. The interplay between those is what sets the winning team apart and makes frontlines fun. If you can get good battlehigh and good score - the 2nd and 3rd should make your life a living hell. Of course as some mention 1st would opportunity to retreat and strike back, but 3rd and 2nd also have that opportunity to counter.

Additionally if the strongest team is actually in 3rd and spent the whole game farming BH, you also need to be mindful that they can turn the game around by strength. This is the fun bit!

I like that it serves as a kind of a memory of your performance within a game - if you and your team are doing well - you get rewarded within the game. It encourages PVP and without battlehigh you could just all sit still and wait for nodes - and it would basically be the same. Without BH - there is no frontlines, IMO.

What would I change in BH?

Hm, I think BH system is fine overall, but it's just a multiplier on the effects that were broken to begin with.
What isn't fine is the amount of burst damage and crowd control we have in the game now, which seems to be what a lot of commenters complain about. In burst focussed AOE gameplay your health can go from 100% to 0% within a split second, and I guess that's not fun for a lot of people, as with good BH that can happen with less effort. The skill effects aren't balanced for frontline - they are balanced for a mode with way less players - CC. This is the problem I hope they will address in 7.2 (cope?)

You can also say it's a skill issue, and be somewhat right, because you can definitely watch for the burst and try to counter burst it.