What it does is make it impossible for a new user to verify that the chain followed the rules at the beginning. You and I know (or at least we reasonably believe) that there wasn't any funny business in BBR at the beginning because we were around before during and after the trimming, but a new user starting a node has no way to establish that at all other than trusting you and me or someone like us to tell them that we were around and everything is okay.
That is unlike Bitcoin or Monero where you as a new user, on your own, can literally check every single transaction back to the genesis block and don't need to trust anyone to tell you they all followed the rules. I think the issue is somewhat worse with an anonymous coin than Bitcoin because rule violations can be hidden in the anonymity (if someone has a million extra coins, you couldn't see that or prove it). Nevertheless it isn't is devastating flaw or anything, just a somewhat higher level of external trust required than other coins (with the offsetting benefit of a smaller chain).
I just put non-pruned version of boolberry blockchain: http://boolberry.com/downloads.html#blockchain_proof, special for you, because you like to confuse people with speculations about this stuff.
You can do it for BBR as well, using this full version of blockchain you can mathematically prove that all transactions followed network rules.
The worse that we already discussed it with smooth and he knows that he is wrong here.
Zoidberg
ps: i wondering, when you guys finally stop confuse people about my project ?!