You have to be seriously naive if you think that the opposition today should be the same as the opposition before, where they cooperated (somewhat) and tried to stay out of each others way, which is much easier to do because there's a lot less candidates back then compared to the number of seats available.
Times are different now, and the whole of Singapore is being competed for, and there are a lot more potential opposition candidates now. The most likely way for Singapore opposition to move forward is no longer to have several loose parties, but to have a dominant opposition party like in the majority of democratic countries out there that are in two (or three in some cases) party system.
It is therefore now a race between the opposition party to become the dominant opposition party, and WP is in the lead now, with NSP and SDP behind them. It is therefore no longer in WP's interest to "give way" to the other parties when choosing the constituencies that they want to compete in. Whatever they choose to do is part of their longterm strategy for GE2016 and beyond. There is no point trying to guilt WP into giving way by saying that three-corner fights only help the PAP - while this is true for the immediate GE, it is actually in WP's interest to have three-corner fights as part of the longterm strategy. This was seen in Punggol East SMC, with the results showing that WP is the dominant opposition party over SDA, and makes SDA irrelevant to the political scene in Singapore.
Whether this would have happened in MK, no one knows. What is obvious is that WP had the stomach and will to go into a three corner fight, while NSP... doesn't. I don't know, but I think that they may have been disappointed that the 3-corner fight didn't happen, because Singaporeans really need to know about the necessity of tactical voting because that is going to become more relevant in GE2016. The PE2011 results was probably good for WP because it exposed Singaporeans to why tactical voting is necessary.
I'm sorry, but I'm not buying the excuse that you withdrew from MK for the sake of the opposition. This may be true if either NSP or WP had a chance of winning MK, but neither of them did. I'm definitely more inclined to think that you withdrew because you had no stomach to fight and afraid of losing the deposit like what happened to Desmond Lim.