Why not?

The Toyota salesman told me that the Civic Hybrid is "not a true
hybrid." Would a car salesman lie? :)

It's pretty clear you haven't actually driven one.

If you floor it away from the stoplight, the engine will kick in
almost immediately to aid in acceleration. If you ease away slowly,
the engine will only kick on when the batteries need to be charged,
which can occur at pretty much any speed.

Thermal management is part of the system control.  You don't need to
worry about it.

Modern engines don't run rough when cold anyways.  A combination of
precision machining and adaptive computer algorithms keeps them running
correctly at any temperature.  The only difference is that the A/T
computer maintains a narrower RPM range at cold temperatures to avoid
problems with high oil viscosity and thermal stress.

One scary thing about hybrids:  I owned a 2005 Accord Hybrid for a
little while.  It was such a rare car that dealership mechanics needed
to be on the phone with Honda's tech support while working on it.  The
car eventually had to be bought back under the CA lemon law.  If you buy
a Hybrid, make sure a dealership in your area is competent to fix it.  
The advantage of a Prius is that it is more familiar to mechanics.