There are some drawbacks to that. Memory flagged as hotplug can't be used by the kernel itself (because otherwise unplugging the RAM would risk crashing the system). That includes things like network buffers and a number of things which use the total RAM size as a guideline.

There was a bug in a proposed kernel patch a while back, that we were testing. It ended up designating a good chunk of the machine's RAM as hotpluggable. Test suite was passing, but the benchmarks showed a drastic performance drop. Took a while to track down why that was happening.