Technically speaking, our usual political correspondents have it right. But in Singapore, no party (other than PAP) can field a full slate of candidates. Therefore, the situations portrayed are not that exact when it comes to the oppos.
Contrariwise, these depicted situations reflect the PAP's situations rather accurately.
In the opposition, unless for the really bizarre, many people can be candidates. It depends on whether they are candidates in a "normal" place, or a more "favourable" place with a tenure of vice chairman or CEC member or youthwing president within the 5 years thrown in.
Some people can live with (1). After all if I win I become MP, doesn't matter if I hold any posts.
Some thrive on (2) and would go to a smaller party that gives 2 even if it reduces my chance of winning. I get to charter the entire direction and strategy as well. For that matter, as a leader of a smaller party if I become one, my winning chances are equal to a secondary team in a more popular party, so why not.
I would think Brocoli is right - if GMS, CTL, Gomez stayed with WP, they would be candidates in 2016, other factors aside. There is no reason why not since WP is still far from 87 candidates target. And why did WP from 2006 to 2011 only add 3 more to the slate, if it weren't partly due to turnover.
In opposition politics, there is everything to lose and nothing to gain. Leaving shouldn't be seen as peculiar or necessarily being disfavoured.