SPAN Aggregation question - using spare switch vs. Gigamon, Ixia, etc.

For basic span aggregation needs, why would someone opt for a Gigamon, Ixia or similar when you can use a spare switch to ingest SPAN's from multiple switches and then feed those source interfaces to a destination port which contains your monitoring tool? I realize Gigamon, Ixia, etc. have advanced filtering options such as only feed http to Tool-1, ftp to Tool-2, and the config options of "one to many", "many to one", etc., but if you just have basic needs and want to consolidate and send all data from multiple feeds to a single tool, is there any reason not to use a spare switch?