The requirement for CFIs (in airplanes) to have an instrument rating
has nothing to do with the requirement of CFIs to have a commercial (or
higher) certificate. It is stated right in FAR 61.183 that to be
eligible for a CFI certificate with an airplane, powered lift or
instrument rating you must have an instrument rating (or instrument
privileges) on your pilot certificate. The instrument rating
requirement is there, presumably, so the instructor can get you out of
a jam if you accidentally get into IMC. It is of course possible to be
a helicopter CFI without having an instrument rating.

It is also not necessarily true that a CFII or an MEI is better than a
"basic" CFI. From the FAA perspective there is no such thing as a
"basic" CFI, only a flight instructor certificate, and ratings that can
go on that certificate. While the usual progression is: Airplane
Single Engine, Instrument Airplane, Airplane Multi-Engine; The
instructor ratings can actually be earned in any order. That is, it is
possible, although not common, to get a flight instructor certificate
with an "Instrument Airplane" rating but not an "Airplane Single
Engine" rating. Such an instructor could give training toward an
instrument rating, but could not give training toward a private or
commercial certificate.

One advantage of doing the CFI-I rating first is that when you later
train for you airplane single engine rating, you can do all of the
training and the checkride in a non-complex aircraft since you already
demonstrated complex airplane proficiency on the instrument checkride.

Jeff H., CFI ASE/AME/IA

PS Now for the bonus question. How could one get an airplane CFI
certificate without ever flying a retractable gear airplane?