The main reason that many of us end up using Plex is that it's useful in several different ways, and although there are equally good alternatives in each area, it just seems sensible to consolidate on Plex.
I don't really need to index all the media on my network and add metadata - but as long as the capability is there with Plex, why not?
I can get by without transcoding, at least for one or two devices, as long as I'm willing to do some quick conversions when I need to - but as long as Plex is there and it will do transcoding, I guess I don't need to bother. And I keep adding more devices...
There are other ways to browse my media on my iPod Touch or Android tablet, but I bought those Plex apps some time ago when they were on sale for $1.99, so I might as well use them now that they support the Chromecast.
My old Samsung TV happens to have a Plex client app, and so does my Roku media player, so it supports my devices in addition to the Chromecast.
And Plex has a bunch of channels of its own to supplement what I get from other sources, and some of those channels are killer!
So while Plex is far from perfect - there are bugs and the indexing and meta-data sometimes goes wrong - it's hard to find another tool that does so much.