I think Secombe is often underrated in the Goons. As Foolestroupe says, he was the 'straight man' that all the other characters revolved around. If you hear the few episodes made without Secombe present, where he was replaced by other comics, you notice the difference (these were usually because Secombe was ill, or maybe had other commitments).

When Sellers or Milligan was absent and replaced, the difference was not so noticeable, because other actors/comedians would put on the characters' voices and do a reasonable impression of them. 'Neddie Seagoon' was not so easy to imitate, so instead they replaced the character, to the show's detriment.

In one episode, Sellers was absent, and replaced by a few others - none of them individually was able to do all his characters' voices. But in another episode where Milligan was absent at short notice (probably through ill health), Sellers read all Milligan's lines as well as his own, and few people (apart from those present in the studio audience) noticed the difference!