If you're in something other than C or Am, you're supposed to use key signatures to point that out and clean up the notation. C/Am is just the default.
"Standard musical notation is ambiguous when used on guitar", actually it isn't if you mark strings and fingerings as needed. This is widespread practice, each instrument has its own "tweaks" on the basic system. Standard notation in particular is quite hard to work with on guitar, and one can see why so many people just rely on tablature - but ambiguous it's not.
Chord charts may be "concise" but they don't actually convey harmonic structure very well, not least because of the multitude of informal conventions and the ambiguity that these create; figured bass notation (while admittedly not very well known) does a far better job of it overall. For example, "walking" bass lines become immediately apparent in thoroughbass.
Why would tablature be so popular on guitar if standard musical was overwhelmingly better? Tablature is intuitive, expressive and concise when used with guitars. Just as standard musical notation has its strengths so does tablature. Ok, tablature's poor at expressing rhythm but standard musical notation's terrible for transposition. So they're all just different tools with different strengths.
(For that matter, standard musical notation is not even that terrible for transposition once you understand how key signatures work, via the cycle of fifths. It's a bit tedious, but it absolutely can be done; one can even learn to transpose directly on sight. And good tablature notation will of course include accurate info about rhythm, there are quite easy ways of doing this.)
You mean, if you add tablature above each chord (or, effectively, transcribe that information via numbers, etc)? :)
When the parent said standard notation, he meant the one without this augmentation, for which it wasn't quite optimized.
>Standard notation in particular is quite hard to work with on guitar
That's the point - that it's not the use case the notation primarily was designed for. Guess we all agree here, don't we?
Some people seem to think that standard musical notation is "right" and you can't possibly have any alternative notation and yet this is clearly false. There are at least two or three alternative forms of musical notation in wide use today. Most guitarists use tablature notation because it's better suited to that instrument - standard musical notation is less useful on guitar since it doesn't clearly capture the different ways that a single chord can be played. So it makes sense to have a special notation for that instrument.
Secondly chord charts are used by musicians everywhere because they're very concise at conveying the chord structure of music and often that's all the information you need. Standard notation is just way too much information in that case.
The third, less commonly seen form of musical notation is just like the one you've described. It's seen in video games like SingStar[1] and in modern music composition software such as Ableton Live[2] and others. It's used so often in music composition software because as you've noted it's very intuitive and far less cumbersome than standard musical notation. Transpositions become trivial and everything just makes more sense.
[1] https://www.gamesmen.com.au/media/catalog/product/cache/1/im...
[2] http://whereisdarran.com/wp-content/uploads/2014/02/Screen-S...