With beast blood pellet and clothing with high beast hood, the amount of time required to reach the point where your beast meter grants you extra damage is really not that significant, and if you hit the boss immediately again the momentum is usually more than enough to sustain the meter from dropping too fast.
It's not great for all situations yeah, but at the right time in the right moment it can be very, very devastating.
I guess if I were to try and put it into words, my thought on it would be this:
If you have to ask whether or not you should use a Beast build, you probably don't know the game well enough to use a Beast build.
It's more something for people who've beaten the game six ways to Sunday and just want a different way to experience it, rather than a practical way for anyone to actually go through the game normally. (That's kind of a given, though, considering you need to beat the DLC
and most of the optional bosses
and most of the Chalice content to get the weapon runes, and oath you need to even try the build to begin with.)
Maybe not quite a "handicap" build, but probably the closest Bloodborne has to one, emergent gameplay (SL4 runs, etc.) aside.