My in-laws own a pontoon boat and we've fished off it.
Definitely a comfortable way to travel around the lake. Very stable platform too for someone with mobility problems or if there is a group. Haven't tried it but it would probably be good for a bunch of kids too.
The downside for fishing is that you can't get as close to shore, rocks, etc. as you could in a tinny and you won't exactly be able to roar around the lake looking for spots.
A pontoon boat wouldn't be my first choice but it's a solid option if you need a boat for multiple purposes.
My two cents,
Mike