There's nothing wrong at all with using tables when your content is tabular data. The problem is mostly an historical one, dating to before the widespread adoption of CSS: Tables were once the only reliable way to get any sort of layout structure on a page, and so were hacked to accommodate all sorts of layouts through clever use of colspans and rowspans. This was terribly inefficient, but before we had positionable divs, they were the only game in town.
Tables are a tool. I think that most agree that it is best used for grid/tabular data representation.
Tables are similar to a club (caveman-style), it performs best at a particular task, such as knocking-off dinos, but it was quite often used as a hammer, paper-weight, table-leg, etc. Once specific and readily available tools came about, the club's value as a club didn't diminish, just its extended usability did.
CSS provided programmers the ability to not use a club as a wrench, but there will always remain a value for tables until something more efficient and effective comes about for the task that they serve.
Divs are a like a club too, except the dinos are extinct and so they're expected to be generic. Maybe I'm just feeling nostalgic for something that doesn't exist, but even when looking at clean, clever, or well-architected markup I can't help but think it could be much better.
In my own experience I find them to be very inflexible and unpredictable.
There are still scenarios when you need them though, so I wouldn't call them 'poor practice' so much as something that should only be used if it makes sense.
It really depends on what you are trying to do, I don't really know how to answer the question, it is too broad.
But like I said, I don't think tables are 'bad'. I think the main stigma against tables was that back in the old days they were used to layout pages that had column layouts, but I didn't program back then so I don't really know.
GMail uses a table for laying out emails, is that tabular? (Maybe it is, I don't know, which is why I'm asking.)
What if you have an element which has 5 children that are in a row and you want the one in the middle to take up all the remaining width while the rest just take the width they need? I'm actually curious because I haven't figured it out. (Besides using table cells that is)
A list of emails can be thought of as tabular data. In Gmail, you have columns (selection, star, important, participants, subject, and date) with one row per email. That's a table without explicit headers.
Yeah, with current css specs in browsers you need to resort to table-cell stylings like display:table-row and display:table-cell. But that can be done without using actual <table> elements.