I think that optifine is not needed these days in 1.6.4 .
But you will have problem if you have 32bit os and less than 4gb ram.( uhh)
In 1.4.x era fps was problem but not ram.
After 1.4.x FTB Team decided to add more mods to their modpacks which causes high ram usage.
So if you want smooth performances you dont need optifine(except if you have really old pc) , maybe for some options like no void fog or mipmap.
This might be true for you. It is not true for everyone. I cannot run FTB on my laptop without optifine. It's basically unplayable due to lag spikes. I won't say optifine gets rid of them completely...but with optifine it's more like one lag spike every ten minutes instead of ten seconds of smooth play followed by a thirty second lag spike.
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