I might vote with Bret on this one. We do low fidelity. However, when it gets to datatables we find that providing a little more context (i.e. headers and data that makes sense) allows for people to understand what the page will look like. So, often we go into Excel and create the data that we need and then bring it back into Balsamiq.
That said, the devil is in the details. We usually provide 10+ rows of examples in a datatable. There may be 7 or 8 columns. We often use customer location numbers, first name, last name, email, dates, currency amounts. However, the dates, amounts, etc. are very use-case specific. If my client was expecting $1,000 to show up and autogeneration chooses $20,000 that isn’t helpful. So, then it requires parameters for most of the different kinds of auto-fills.
I think this sounds nice but I’m not sure I would use it… even though I need it :-). Perhaps Balsamiq could make it great, but unless it is almost perfect, I am still likely using Excel.