It seems like a lot of wasted space if each index tablespace had to have enough room to reorg its indexes. It is more efficient to use a common work area, like a system temporary tablespace.
If your system temporary tablespace is not large enough to reorg an index, it is likely too small, especially in a data warehouse application. If possible, make the system temporary tablespace at least as large as the largest table that could be scanned with a tablespace scan. If you have multiple simultaneous users scanning a tablespace, it needs to be larger.
Obviously, for SMS tablespaces, you just need sufficient room on the drive for it to expand as needed.