Should I spend thousands of pounds on a 600mm f4 lens that I can hardly lift? The quality is said to be superb.
Let's put a 1.4x converter on my ancient 300mm f4L (making it 420mm) then put it on an EOS 7d body, which having an smaller APS sensor magnifies everything by 1.6x anyway making 670mm. Mmm, not bad; I can hold it in one hand, and with a short monopod stuck in my belt I have a fairly stable hand held 670mm f5.6.
Down by the river this morning a Moorhen was feeding her chicks, so I tried it out: (click to enlarge)