I presently have four fixed gear conversions. None of them would be "better" than a proper track bike except that:

1. If you want to ride fixed on a bike with traditional road geometry, a conversion is probably better because most track frames have track geometry.

2. If you want to ride fixed on the road, you might want more than one water bottle cage. You might even want to install a rack and fenders. Very few track frames have the required eyelets and braze-ons to do that.

3. Building a fixed gear out of an old road bike helps keep old bike frames from being melted down and made into Cadillac Escalades and M16 rifles.

4. My four fixed gear frames have cost me $0, $35, $0 and $75. It's very hard to find track frames on ebay or Craigslist that cheap. For those prices, I can live without track fork ends.