imho, this post gives an interesting point of view on merged mining:

http://blog.melotic.com/2014/09/30/the-scrypt-alliance/

I do not have enough understanding as to the impact of merged mining on a coin.
I can say though that my jut feeling nd limited view is that we are offered/offering the following tradeoff -
1) we help Litecoin hedge its #1 alt position
2) in exchange, we get 'some' security against DDOS

However, i'm into Viacoin because I believe it can overtake in significance and impact most other alts [certainly litecoin], and help disrupt the market in a good way. If merged mining is going to give us 'some' security' but make Viacoin less independent, or less flexible - then i'd vote against merged mining.
If Viacoin has a network hashrate problem, I'm sure some prominent bagholders can each take some rigs and help secure the network, as has already been done by a few on this forum.

From BTCdrak's last post, I understand that he is in favour of merged mining... and i'd love to hear his rational as him and PT are the most authorative people on the matter and their view counts imho more than a 100 community members like me that have limited understanding of the topic.
As I said, i'm still trying to understand the concept.