It's considerably more complicated - and much more expensive - to set up to run both systems at the same time, and only a relatively small minority of operators do that, I suspect.
I haven't tried it yet, I recently did some (Iron) Horse trading at my LTS, and Finally got a DCS system. My understanding is that you can simply hook them both up at the same time, the DCS and Legacy/TMCC systems work so much differently from each other, that they do not interfere with each other at all. No more complicated than picking up the appropriate remote for whichever type locomotive you want to run.
Setting up both systems is much less complicated than wiring a decent size layout for block wiring for conventional control. CTT a while back had an article about using both systems, and the way they explained it was overly complicated, and several members posted that the extra wiring the article described was unnecessary.
As for the cost, each system is a fraction of the cost of a single locomotive in many cases. Guys will have several to many $1,000 locomotives, then complain about having two $300 command systems. I enjoy large locomotive rosters as much as anyone else, but for less than the cost of one locomotive, you can get both systems, and in the case of many locomotives an extra remote for each system as well. Those command systems are likely to enhance a layout much more than a single locomotive, unless you have a really small roster.
I suspect there are a lot more people running both systems than you realize.
Doug