I think most people maintain a healthy mix of both. Most relationships at work remain strictly professional (bosses, leaderships, *cough* marketing). But most people I'd guess develop some level of friendship with some peers that you vibe with.
Has your developing a friendship @ work ever backfired? Where you think you're cool then get blindsided by some bullshit? I've witnessed cases where a group of AEs becomes tight, then one AE is promoted to Sales Manager, then they shift their relationship dynamic to be 100% professional. Also seen the reverse, where they keep the friendship, but then makes the working environment more challenging. Also seen it work out great, where the parties can keep things professional when they need to, but also have a healthy friendship outside of the space.
I've had a few instances where I had friendships but for one reason or another, they fizzled away and now have healthy, strictly professional relationships.