for QuickTime will no longer be maintained and updated, in part because (QuickTime) alternatives are available for current OS versions. However, those alternatives may not be available for Leopard, and consequently, as long as you use that, removing Perian may not be a good idea unless an alternative like DIVX Plus can take over. Note that the Perian site also lists alternative players.
If you consider switching players, make sure your choice of player or codec pack can handle the file formats and video types you want to view in an acceptable way
. Both Perian and DivX
sites (scroll down) list the ones they handle. Note that DivX handles fewer than QuickTime w/ Perian, but handling of some formats/types may be faster (e.g., .mkv). Also note that DivX player 9 may have updated codecs for one or some formats (likely DivX itself), but not necessarily for others it handles, and certainly not for those it can't handle. I also can't tell if DivX Player 9 will interfere with QuickTime/Perian playback or vv, so you might want to keep that possibility in mind if you pick DivX but don't want to remove Perian right away.
PS, the DivX Player includes several codecs; I don't believe their codec pack adds anything to that (see the format/feature chart mentioned above), at least not this one
. If anyone feels the need to correct this view, please do, with available details.