The spreadsheet linked in my signature lists the sha256 coins that can be merge mined together. If column I has a number then merge mining is supported (as a child to BTC).
The only coins I mine are SHA-256 with merge mining capabilities.
If you look at the number of alt coins (both sha256 and scrypt) that have come and gone in the past couple of years you'll find that most the sha-256 coins that are capable of being mined as a child coin still have a good number of hashes against them because of merge mining. Every time a new scrypt coin comes out people stop mining one coin to start the next. The older coins eventually die off and anyone who invested any sort of resources into that coin ends up losing in some way.
Now, if scrypt coins were to start appearing with merge mining support that might change my view on things.