I didn't say Fuente was better than Merckx. I said he was better than Merckx in a particular race to which I was referring. That race is the 1974 Giro, when Fuente lost ten minutes into Sanremo on stage 14 yet still finished just 3'22" off Merckx, winning 5 stages - all lumpy, mostly high mountain - in the process.
Being better than Merckx is an impressive feat, although Fuente wasn't as consistent as Merckx, had too many other weaknesses and his heyday was short. However, if Merckx deserves to be in the list, then the guys that outclimbed him sometimes definitely merit consideration. You said that they did not, and therefore you are wrong.
The "pure climbers" often are quite inconsistent - Antón and Rodríguez in the current péloton, for example. Fuente was consistent enough to win two GTs and 5 KOM jerseys, and this when racing the Vuelta and Giro back to back in the days when the Vuelta was in April. Merckx was a better rider, and often a better climber too. But Fuente was a guy who he truly feared in the mountains. That alone is enough to merit his consideration in a topic about the best CLIMBERS of all time.
The whole thing about GCs and Merckx' other races is valid, but runs counter to the OP's original criteria. Under the original criteria, it is lunacy to state that Van Impe, Fuente, Herrera and Ocaña "don't merit consideration". You may ultimately reject the argument, but to say that they "don't merit consideration" is simply ridiculous.
Hence why I was in favour of 9 options plus a tenth, "other: please specify" category, because there are clearly more than 10 riders in the history of the sport whose achievements in the high mountains "merit consideration" in discussion of who the best climber is. I don't necessarily think that José Manuel Fuente is the best climber of all time. But I do think he is in the top 10.