Why we always balance system chunk alone with metadata?

[Qu Wenruo <quwenruo@cn.fujitsu.com>]

Hi,

During debugging a bug related to balancing metadata chunk, we found 
that if we specify -m option for "btrfs balance", it will always balance 
system chunk too.

cmds-balance.c:
---
         /*
          * allow -s only under --force, otherwise do with system chunks
          * the same thing we were ordered to do with meta chunks
          */
         if (args.flags & BTRFS_BALANCE_SYSTEM) {
                 if (!force) {
                         error(
                             "Refusing to explicitly operate on system 
chunks.\n"
                             "Pass --force if you really want to do that.");
                         return 1;
                 }
         } else if (args.flags & BTRFS_BALANCE_METADATA) {
                 args.flags |= BTRFS_BALANCE_SYSTEM; <<< Here
                 memcpy(&args.sys, &args.meta,
                         sizeof(struct btrfs_balance_args));
         }
---

I'm curious why we always bind system chunk to metadata balance?

Is there any special reason?
The patch introducing such behavior is dated back to 2012, and this 
makes us unable to do metadata *only* balance.

Any idea?

Thanks,
Qu