I can see ONE good reason to have Symbian, at least for some time, on the tablet :
Nokias S60 ser.3 provides a relly good PIM app. together with a search pgm. (Nokias "Search") that can find any text in the PIM database, e-mails and messages plus any file name in the file system.

( By the way, if you have a Symbian device, get the program "X-plore" from www.lonelycatgames.com, a good file manager + text editor, even on the small screen of the Nokia 5500. )

There is of course Garnet-Palm, but not quite the real thing with the N810 keyboard as the Palm screen works better in portrait mode.

Except for this, I quite agree that Symbians time is really over.
I read somewhere that Symbian is a nuisance to program in, as many of its structures come from the time when memory was scarce and every saved byte of space was essential.

So, unless porting Symbian to the tablet is really simple, programming efforts are probably better invested in bettering PIMs for Mer (or Freemantle, if you consider the next? Nokia).