Combat Trim system makes perfect sense.
The instances where people use trims manually are usually limited to certain situations or certain planes. It's not like you're going to have to keep trimming your plane at every given situation in combat.. it isn't possible anyway.
When you need it, you can go manual for maximum efficiency. Otherwise, it really isn't all that important - the only difference is something like "when you want a sustained climb angle do you press shift+X, or do you set it manually?"
IL-2 is the exact opposite in that people have to trim their planes in the situations when it is hardly appropriate. The trim effects the performance of the plane so harshly that a bad set of trims will frustrate your every attempt at aiming at the target. The way trim is modelled there also advocates an exploit known as "bat turns", too.
So, in this case, whether you go manual, or combat trim, it is entirely up to your preference. Just be sure to use manual when you really, really need it, and you're OK.