Notso,
I believe the very fact that the airbus engineers managed to convince the airbus and B.A. bean-counters on the cost/benefit issue answers the worthiness of winglets. However, I do agree with you that the efficiency of winglets reduces with range, but to counteract this, de-rated take-offs, and hence savings in fuel and engine life cycles, must be the overriding factor.
RE: G-EUPA's situation - I imagine as soon as spares become available, and a suitable slot time in her schedule arises, she will have a pair of winglets fitted. Time will tell.
Regards,
VC10 Rib22
ps although this is a different aircraft, I'm sure the the theory is similar
http://www.b737.org.uk/winglets.htm 