Transponder requirement confusion

In article ,
wrote:

Quick (hopefully) question to a confusing part of the FARs. With my new (to
me) solid-state transponder, I'm now willing to power it up for even local
VFR
flights. Before, I would only turn on my KT-76 if I needed to (VFR
advisories, IFR,
Class B/C operation) since I knew the cavity tube had a finite number of
hours before
it died. Now that I read through the transponder requirement carefully, it
appears
that that's not allowed. Google groups appear to contain previous usenet
arguments
regarding this, but the answer's still not quite clear. In particular:

FAR 91.215(5)(c)
(c) Transponder-on operation. While in the airspace as specified in paragraph
(b) of
this section or in all controlled airspace, each person operating an aircraft
equipped
with an operable ATC transponder maintained in accordance with ??91.413 of
this part
shall operate the transponder, including Mode C equipment if installed, and
shall
reply on the appropriate code or as assigned by ATC.

"Controlled" airspace also includes Class E... so if you have one (and it's
within the 2-year test), you *have* to turn it on? ... but you aren't
*required* to
have one?

If so, that's a very unclear requirement from most colloquial references on
equipment requirements...

The really dumb part of that requirement is that terminal radar doesn't
see below a given altitude at a given distance. Our local site (900 MSL)
cannot see aircraft with transponders on below (5000 MSL) at airports 40
south of the radar site. It gets really exciting when they are
controlling you into that airport on an instrument approach.