The BD bikes are an amazing deal IF you want to deal with putting a bike together, securing your cog/ lockring/ truing wheels out the box, etc.

If you're brand new to bikes, I think you should definitely go to a shop, get sized up, and pick up a Madison/ Lager/ etc. Have them show you how the cog works, swap the stem for a proper fit, get them to swap out to the bars you want for a discount, get a follow-up wheel true in a month, go back for any questions you might have. All this is worth the $100 that BD saves, if you don't have the skills/ knowledge yet to do it.

As far as it goes, I think people offer the BD advice because they've learned the hard way that doing a conversion gets kind of expensive pretty quick, to the point where it makes way more sense to buy something with all brand new parts that are built to work together, even if they're not the highest quality. And at the end of the day the conversion will be an old frame that doesn't have track geometry.

I ride a conversion, but I've worked at bike co-ops and bike shops.