There's also the adoption perspective. If there was a cryptocurrency with the level of adoption/network effects like Dogecoin and its mature market, that would probably be a very good merge mine coin.
Yeah, I suppose that is one of the reasons why Doge is being merge mined by LTC miners (they both use the same scrypt algorithm for their PoW). But I think for miners the main incentive is still profitability, I doubt majority of miners care about popularity of an altcoin. Although the two are not mutually exclusive, a popular coin can be more profitable as well.