I think 30 km is a pretty long hike to be considered a normal part of a bike tour, so I consider it a distance that is worthy of shoes specific to the task. I tend to pack crazy light (8-14 pounds base gear weight is normal for me these days), but I'd wear my bike shoes on the bike and take some light trail runners for the hikes if those hikes will be more than a few miles.

It probably boils down to how much focus each activity has on the trip. Most tours I focus on the riding and minimize the hiking. For that I make do with bike shoes for the short hikes. If you are talking 30 km hikes I would consider that more of a dual mode trip and take more optimum footwear for both. If you prefer to pedal in "normal" shoes I guess the choice might be the same for both, but it wouldn't for me. I'd probably bite the bullet on the extra pair of shoes, after all you can still go pretty light if you take a minimalist approach on everything else.

One other thought... My usual approach is to limit any hikes to a couple miles or a bit more, but on one tour I bought a pair of light trail runners when I stopped for a week of sightseeing and hiking in Yosemite. Would this 30 km hike be in one particular spot on your tour? If so you might consider trying to figure out how to have extra shoes only for that portion of the trip. Buy them mid trip? Mail them to yourself? Mail them home?