How would you label requests for substitution during the contract administration phase?

o A necessary evil
o A potential risk issue
o A compromise of finish quality
o A way for contractors to increase profits
o A time-wasting, profit-munching pain in the backside
o All of the above

YES! There is IS something you can do to tame this beast – in fact to just make it go away.
There is one (and only one) condition where you are obliged to process a request for substitution without getting paid for it, and the condition is one of the following: when you have specified a product that is (a) no longer available, (b) simply won't work as designed, or (c) cannot be obtained within a time period that won't delay construction. Of course, you should not do those things anyway but sometimes it just happens, especially when you are in the habit of producing fake specs.
It is theoretically possible that a most benevolent contractor has located a substitute product that really is better, and less expensive, than the one you specified. However, the incidence of this happening is extremely rare. If it does happen, you should gratefully review the request.

In the vast majority of cases, the request for substitution is labelled "all of the above" (in the quiz above) and, in particular, can lead to a diminution of finish quality, increase your risk, and cause you to spend unpaid professional time processing it.
Many practices have tried to banish substitutions by declaring in the spec that none will be considered. This strategy fails more often than it succeeds – contractors gleefully end-run it with a variety of tactics.

There are two approaches to sharply reduce the number of requests:
First: Add clauses to the Owner-Architect agreement that state that requests for substitution will be (a) referred to the client for review approval, and (b) if approved, be processed as Additional Services. The contractor still has some tactics here, and will try to show that the product you specified is not available in time, etc. etc.
You can up the ante on this strategy with a clause in the construction contract that provides that any requests for substitution (except where the architect has specified unavailable products) will be processed at a fixed charge, deducted from the contractor's retention as a deduct change order, and that this charge will apply regardless of whether the request is approved or not. The money saved can then be transferred to you by the owner under appropriate contract language. The contractor still has some tactics available here, but will think twice about lobbing cheaper products in your direction.
Second: There is a really great solution out there – one that I learned from Tulsa architect Charles Chief Boyd many years ago. His solution, which he gave me permission to distribute, requires the contractor to do about 95% of the work that processing normally takes. Chief Boyd developed a two-page form that must be completed in order to request a substitution. Just click here to view a copy. You also need an enabling clause in your specifications, which is found on the third page.