Sara is a hardworking player, with a great forehand, net skills, and touch. The fact that she primarily relies on her fitness and topspin hardly makes her a bad player. Honestly, she should be respected for making the most out of her skill set. I compare her to Cibulkova, who is hit/miss and has had some great results, but nothing close to the consistency Errani has. I'd take Sara's career any day of the week.
The entire premise of the thread is retarded. Reaching top 10 is a huge achievement for a professional tennis player (you have to have at least one very good season to just get there), and trying to pinpoint the worst one in history is like, IDK, choosing the worst chocolate amongst the elite brands. They all taste good at the end of the day.
As for specifically Errani, she cannot be titled the worst top 10 even if one was stubborn enough to make such a contest:
a) she backed up her sudden ascension to top 10 with solid results ever since - some top 10 players in history climbed to the outer edge of top 10 and dropped off immediately, never to be seen up there again
b) there were times with much worse depth of field
c) in general, players at the beginnings of the open era did not conduct as professionally as the modern players do. Navratilova's and Lendl's fitness regimes were novelties back in their days, nowadays it's a standard and Errani managed to shine, so she would most likely beat many players of yesteryear
d) Errani's an amazingly successful doubles player, which shows she has many great tennis traits outside baseline grinding