This gives them a somehow secured blockchain at almost no cost, the downside, all these coins are worth little to nothing, so the rewards are almost nothing, I just checked my mining account on Viabtc (which I longer use), and it has 2.5BTC cumulative mining rewards, some NMC, SYS, EMC and ELA, the current value of all these merged mined coins is below $20, maybe at some point in the past bull cycle they were worth $100, but this was obtained after years of mining on the same pool, would I take $10 for every 1BTC mined? why not, would I bother switching pools just to get them? probably not.
I don't know if double mining negatively affects bitcoin mining, but if it does not affect these coins can be mined and stored in the long term (at least in theory), for example ELA is now $0.8 while its ATH was about $80, also SYS is now priced 0.09 while its ATH was around $1.
I know these coins don't necessarily have to get their ATH back but my point is they are not profitable right now but they could be profitable in the long run.