The main reason, if you vent after your venting air that has already been cooled and that gets replaced with more warm air.
Having to remove extra heat is always something you want to avoid.
The problem is the air volume that has to be exhausted is more when it's hot than when it's cold. This is a common problem with industrial turbos where one side BOV works after the cooler, but it must be 25 to 50% larger when placed upstream of the cooler.
Placing it there will work, but only if the additional volume of the hot air being relieved is taken into account. Otherwise, this can cause a surge issue due to inadequate relief valve sizing.
Unless your car completely stops airflow across the intercooler when the BOV opens, it's a spurious argument to justify placing the BOV before the I/C as opposed to after it.
Additionally, the reverse wave of pressure will start at the T/B, and a BOV will stop this from travelling upstream towards the turbocharger. Your flow remains 'towards' the T/B when it vents there, venting at the turbocharger will allow reverse flow through the entire system before relief...
This can manifest itself in transitional response issues.
Edited by Tony D, 18 November 2010 - 11:33 PM.
Misanthropic Anthroparion Class 5 Hoarder, aspiring to posthumous fame as my containers are cut open and the market floods with crap I've squirrelled away over the years! I endeavour to persevere...