1. I suspect with merged mining miners constantly dump (some) coins because they obtain them as a free bonus. Of course this happens to less important, unwanted, minor coins.
(Dogecoin price movement was specific. doge security was low and the trend was bad - price was depressed especially because of that. Merged mining improved hash rate, so people think it's more secure now.)
2. I also heard about possible attacks of majority miners on minor coins (when merged mined).
But I don't know that much about merged mining..I would respect dev's opinion
Regarding point 2. This is easy to imagine if you think about Bitcoin and the mining pools diversity (well lack thereof) -
https://blockchain.info/pools. Now imagine you bootstrap a coin using merged mining and it has a few solo miners and then one of those large pools. One of those pools will have majority hash power. Merged mining's risk is that you are mining child coins for free, there is no cost. Normally to mine bitcoin you expend energy and you have to chose a block to mine on, and every second you are mining you are expending resources. The merged mining part is free, there is no additional cost. So, if a pool wanted to attack a coin, it can do so without hurting it's bitcoin revenue since it's not switching to mine a separate coin. So the risk is during bootstrapping, if you dont have enough diversity of large pools, AND a pool wanted to attack the coin, then it would be game over. This isnt theory, it happened at least once. That assumes more of a malicious incentive than economic. If you add sidechains into the equation, you give a strong financial incentive to attack and double spend the chain.
For Viacoin I believe it's different because for a start, we're using scrypt PoW and mining diversity is pretty good, meaning there are a lot of scrypt mining pools. It is easy to begin merged mining with the support of several pools to ensure that the hash rate on merged mining would be sufficient as to instantly secure the chain against any potential malicious pool. Since scrypt ASICs are not orders of magnitude more powerful, the transition to higher hash rates would not jump exponentially.
We've seen two examples of successfully merged mined hard forks, Doge and Sys. I have already spoken to some pools out of interest and they would be interested in merged mining. I believe merged mining would give us more benefits than there are risks and in the end increases security. You can see the effect on Doge on this graph (select the ALL timeframe top left)
http://www.coinwarz.com/network-hashrate-charts/dogecoin-network-hashrate-chart