The main point to consider is your scalability and reliability over the lifecycle of the application. Of course, as programmers we know that nothing is “cemented” in place, but the effort we’ve spent on core development is often hard to offset while we’re ramping up for a growth curve. If there is anything about your core that could limit your ability to implement new customer instances transparently, to operate and maintain the application without downtime – it is a constaint you may find you cannot afford. There is nothing inherent in a LAMP approach that would limit a company but those that use it often find it difficult to make business tradeoffs to lower development effort and bring a solid application to market.