If I may...
I will answer in my way, using my long web expirence. In general, the fonts are chosen by the client even before the site is built. Logos, colors, design are all established as part of the corporate identity (CI). On the other hand, if you redesign someone's site then you also know the fonts that were used. Therefore, there is no need to guess. If someone plays with the choice of fonts during the construction of the site - it certainly means that he is making a private site. Or makes a site for a woman (30+) who changes her mind every few hours, brrr ;/
Yes a live preview would be useful and we will certainly implement this one day. For now, however, it is worth enjoying what is already there :)