Lets say I have a MyISAM table (amongst other tables) with 2938347 rows. This table is merely being used for logging and doesn't get emptied very often. 

By not regularly archiving or deleting these rows, does having this table around affect server performance?
Then, what about tables/databases on a MySQL server that are massive in size, but aren't accessed all that often. These tables co-exist with frequently accessed/queried tables, and I'm wondering if they're causing problems or not.

Darius