Purpose of using same speakers for surround?

Not one manufacturer has yet to produce a completely neutral speaker. Not only do different designer/manufactures deviate from neutrality differently, but even models from the same manufacturer deviate differently from each other. With that said; it is more likely that a given designer/manufacturer will more likely have more consistent design priorities, and even perhaps shared components within a given line, and therefore greater sound similarities. There is good reason to match sounds as closely as possible. Different variances from neutrality can add quite a bit of distraction from the semblance of reality. While perhaps not as important as matching left and right channels, the same reasoning for doing so, holds true for matching front and rear channels.

Edorr, your dismissal of the previous advice and subsequently offered advice, is predicated on assumptions and qualifications that have not been provided by the OP. Even if what you suggested were true for the OP, very often the budget allocation could permit the less expensive surrounds to come from the same manufacturer, which would more often than not, be a better match with the mains.

Edorr, I suggest you read past the title of OP opening thread opening. The other obvious exception is for those that have separate audio and HT rooms.

IMO, for not making a major investment, I feel the change would be a significant improvement and worth the effort.