I'm not sure where this religious insistence to BBUs comes from. Of course a hardware RAID controller can be a "go" even if it lacks a BBU, especially if the user has cost considerations.
BBUs are meant to protect the write cache from a power failure, so when the server loses power, the cache retains the written data (and when the array comes online again, it will complete the write).
But you know what else is meant to protect your data from a power failure? A decent journaling filesystem like ext3 or ext4, with sensible mount options (ordered data and write barriers), which Proxmox already uses. Sure you will probably lose the contents of the write cache, but your filesystem will be consistent, and your data most likely intact (albeit at a couple of seconds older state) at a power failure. Also, don't forget that despite having a BBU, you will still have to do an array consistency check after a real power failure, as your disks sure have lost power...
Not to mention the fact that in our many years of being in data center, we NEVER had an unannounced power outage, only a single PSU failure many years back (which all the data survived just fine, thanks to ext3). The only time we REALLY lost data was when we used mdraid and a kernel panic actually FUBAR'd the array - which NEVER happened with the Adaptec controllers, even though we had dozens of hard resets due to the unstable kernels.
So when a user has to decide between using mdraid and an affordable hardware RAID controller without BBU, the sensible advice is get the controller. BBU is something you want to put in an otherwise fully redundant server (like having at least 2 PSUs), but is far from being the most important consideration here.
