Buying more cable and having to climb the mast are not really valid reasons to be against placing it up there, it is a one time job. You have to buy some sort of mount either way.
I'd have to buy a stern pole anyway for my wind generator (looking at a KISS right now, wife is from Trinidad and will bring it back for me). But I generally disagree with that statement above.
The Radome cost me less than $900 new from West Marine on sale. With a mast mount, you're talking about a mounting bracket/contraption ($300) and an extra length of cable from Raymarine ($200 again). Then going up the mast is a rare treat - fraught with risk of its own. It may be one time costs, but then an extra 50% in parts is a tough pill...not to mention the difficulty in diagnosing something when it goes wrong. The stern pole would be much easier to issue diagnose/repair too.