lowering springs without properly matched struts can/will ruin the struts.
The OEM struts are quality, and arent just going to "blow out" after slapping in some lowering springs.
What wears them out is after months or years of driving the car, the struts will wear out and become bouncy. This is dependant upon driving style.
You MIGHT be able to get away with NOT repalcing the struts..
but in the end, suspension is a WEAR part, and "improperly" modding your suspension - i.e. springs and no struts, will just wear your struts out even faster.
2008 328i E90 Jet Black:6MT:Premium:Sport:Walnut/Black interior:Xenon:MTEC V3 AEs:19" Beyern Mesh:35% Tint:Blacklines:OEM Front Splitters