I thought that getmemorypool was the only thing necessary for merge mining to work. And i've been using 0.5.1 since 2 months for solo-mm.
Bitcoin only has the code for being the PRIMARY chain in merged mining.
In order to be an AUXILIARY chain, you need more code, which bitcoin itself doesn't seem to want to have, presumably due to a desire never to play second fiddle to some other chain(s).
To update an altcoin to be based on the latest bitcoin code, or to start a new altcoin based on latest bitcoin code, this patch is useful if you want to be able to merged-min your altcoin as an aux chain. If you base it on bitcoin code without adding this, like RUcoin did, you have to be the primary chain, which means you will not be able to include bitcoin itself into your merged mining mix.
I made this patch because there are many altcoins that could use an upgrade to the latest bitcoin code but that people mine as aux chains.
So basically I plan to take ths "newcoin" template and use it to make an updated, fully merged-mine-able altcoin, and then make another patch showing exactly what has to be changed when turning a bitcoin that has the above patch applied into an actual different coinchain.
Ny example actual different coin chain will probably be "PRGcoin", short for "PRoGressivecoin", which will be a coin whose block reward starts at one coin per block then increases by one every [some number of blocks].
That will give me a diff showing all the places where changes have to be made when making a minally different from bitcoin altcoin.
I will then be able to fairly rapidly make updates for all the altcoins that are basically hardly any different from bitcoin.