if you cant deduce a songs rythm/ timing from a ascii tab I think you need to practice your instrument some more tbh
Well, that's not the problem. But lets look for example the Skvate's Apex tab: he has put all the notes just one after another, no note durations (the tab is otherwise good

).
It's a bit slow to try to learn the song without listening the song over and over to hear which notes play longer/shorter than the others. This is REALLY annoying especially in solos. But if you have decent GP/PT tab you can hear straight away how does the solo go, or even use speed trainer to play it on smaller tempo.
AND if you have good PT/GP tab, you can start rehearsing the song immediately. Sometimes even without listening the song. Lets say I have a GP tab with 2 gtrs, bass, and drums, heck I don't even need the original song to practise: I just mute one of the guitars and start playing along.
However, I have seen very good ASCII-tabs and I have used them many times. But now when I'm have found PT&GP, it's hard to return to the old times... Especially when most of the ASCII tabs are either tabbed wrong or incomplete or there is only one guitar.
GP/PT tabs are seldom incomplete and usually quite right since e.g. Powertab.net won't accept too incomplete or wrong tabs.