I aassume you are talking about migrating from MobileMe? And that despite what your signature says you do actually have, or are intending to get, Lion 10.7.2 as required by iCloud?
The downside of not migrating is that you will not be able to access your email, and your calendar, contacts and bookmarks syncing will stop. Given that, there is no downside to migrating.
Your iDisk, together with the Gallery and any website hosting, will cease anyway at the end of June whether you migrate or not. so you will need to be looking for a third party alternative. This page examines some options:
You should be aware that if you do not yet have Lion and are planning to upgrade, that in the absence of Rosetta PPC programs will not work.
If you cannot upgrade your PPC Apps, but must migrate to Lion, consider using Snow Leopard in Parallels 7 in Lion:
Full installation instructions for Snow Leopard in Parallels 7 in Lion: