It's actually quite useful to compare to Bitcoins. Most people use this measure as an indication of how good coins are to mine and as it's easily the most recognizable cryptocurrency, it's easy for people to understand. You can compare all SHA26 and Scrypt coins to either Bitcoin or Litecoin in terms of profitability. If you check a site like coinchoose.com, you can see that. It would really only be helpful to compare to Litecoin profitability in cases were coins can only be exchanged for Litecoins and not Bitcoins, but that is a pretty rare case.

Besides, Bitcoin is the coin with the most liquidity in terms of fiat currency, which is what most people are working towards anyway.