Are expired patents still prior art?

The fact that a patent has lapsed has no bearing as to whether or not a patent is prior art - a patent may not be prior art depending upon its publication date relative to, for example, the filing date of a US application - if, for example, a US application is filed January 15, 2015, and a foreign patent has a publication date prior to that date, it is prior art. If the publication date is after the US filing date, even if it is before the publication date of the US application, it is not prior art. There are many other scenarios where some patent or patent application publication would or would not be considered prior art based upon filing and publication dates. Ordinarily, any patent, lapsed, still in force, whatever, can be prior art and someone cannot come along and apply for a patent with respect to that same technology. Actually, they could apply for a patent, but it would not be granted, or if granted, it would be invalidated if the claimed technology in the later patent is truly disclosed within the lapsed patent.
As for a strategic reason to let a patent lapse/expire, I cannot think of any - normally, patents are permitted to lapse or expire because the patent owner no longer wants to pay the annuities or maintenance fees - sometimes, the patent owner feels the fees are excessive/too expensive, or that it is simply not worth maintaining the patent in force because the patent is not being worked - the invention is not being practiced, licensed, or whatever.

At Robrecht Dumarey - please note that "patents" never expire or are abandoned before or without any publication - patent "applications" may be abandoned before publication, or provisional patent applications may never be made public, or accessible to the public, if they are not perfected into non-provisional or PCT applications. Patents, effectively by definition, are always published or made public, or are accessible to the public.