You can solo almost every area with most builds, doesn't mean it's necessarily fun or easy... in this case low will save and bards that can spam dispels mean rogues are going to have to at minimum be quick with pfe wands or deal with wandering around confused now and then.
Weapon Master is not all fighter builds and by ignoring WM damage spike (which can be fun but unnecessary) fighters can instead take prcs that make them end up with low to mid 30s for fort and will plus 20s reflex.
But to stay on topic, my point was simply that having dcs that require some investment in rogue or a ton of Skeleton keys plus Spamming dispels and various other enemy types makes for a very good reason to party of there were more encounter triggers.