The Canon TC80N3 is more suited for astrophotography. You really don't want to press and hold, you want to make many exposures, say 30 seconds to 2 minutes or more and then stack them using Registax or other type of software.
I've dabble a bit in astrophotography but quickly gave it up. Here is a stack of 9 exposures, 2 minutes each, of M42.
and this is a stack of 9 at 1/250th of a second of the moon.
The timer really comes into play when you want to do something a bit tricky. Here, I have captured the motion of the asteroid Ceres.
To get this shot I took 15 exposures (probably 15-30 seconds) every half hour. The stacked them all an the motion of Ceres is clearly visible. It would be difficult to do this without an automatic timer.
For all these shots I programmed the timer, set the scope going for however many hours it took, then either went inside to get warm (for Orion) or watch the Red Sox (for Ceres) or I just stayed outside and did binocular astronomy. M42 and Ceres were captured with a Burgess 1278 (127mm f8) telescope, the moon was probably with a Celestron 8" (2000mm at f10). Both scopes were mounted on a Celestron tracking mount. The camera was a Canon 40D modified for astrophotography by removing the glass infrared filter.