Does transport matter if using external DAC?

In theory, you're missing the fact that the ones and zeros are encoded at precisely timed intervals. If the transport fails to convey the precise timing of the data to the DAC a phenomenom called jitter occurs. For credible evidence read the jitter specs published during transport tests.

In practice, you're missing the fact that the quality of design thought, the parts used, their layout and vibrational issues all affect any machine's ability to produce the results desired. For credible evidence test drive a corvette.

Or rely on anecdotal evidence: my CD player with digital out sounded like it was creating a gap between the sound of cymbals and the rest of the audio stream. They sounded disjointed and in that sense highlighted. Installing a dedicated transport eliminated the gap. The sounds of cymbals became seamless and integrated into the whole. Was this jitter or some other quality of implementation? Who knows for sure.

Ones and zeros. Once again a little knowledge is a dangerous thing. As with digital cables, transports matter greatly.

No question about it, makes a big difference, my CEC took things to a whole new dimension. Also, the choice of digital cable will play a part in this as well.

Rock Virgo has explained things well; it ain't just an issue of ones and zeros.
Also, if you scroll down just a bit on this Digital Forum, you'll find threads with people asking about which transports are best. There's an obvious inference you can draw from that, plus info for you in the threads too.

A great bunch of answers, a pleasure to learn from.

I've owned four one-box players, two transports and two DACs, and I've used one of the players as a transport. My most recent DAC was a clear step up, first with a player as transport, then with a pure transport once I had sold the player. The final configuration was much the best, I thought. In other words, the transport made a great difference, in speed, articulation and soundstage, in this case.

Digital cables also make a great difference, but if you limit yourself to considering ones and zeros then cables are voodoo too.

If differences can be perceived, but they don't show up on instruments, then we haven't learned what or how to measure yet. Just yesterday I read on Richard Sequerra's site a wonderfully lucid explanation of why and how "inaudible" frequencies, by intermodulation, affect what we hear.