Agreed, you'd probably be better off with Bridge Mode for that reason. In the guest case, when it does scale very high, that's when you might see that Guest SSID also use Bridge mode, but leveraging a dedicated Guest VLAN on the wired side.
Since it was a pro/con type question, one thing I left out was the basic adult content filtering that's there when running in NAT mode, you wouldn't have that in Bridge mode. But as you mentioned if you have an MX appliance that's the ideal way to have full content filtering and traffic shaping (plus FW, IDS/IPS, AMP, VPN). On a larger scale, you have that option of a distributed traffic shaping solution by enforcing those rules right at the edge of the network on the APs, sparing the wired infrastructure from traffic storms and placing all the traffic shaping and firewalling burden on the dedicated appliance.
I've yet to see a deployment where a very high number of either traffic shaping rules and/or firewall rules on the APs caused noticeable performance or latency problems for clients, so long as best & common practices were implemented. Meraki APs generally have more CPU and memory packed into them than is necessary in order to accommodate future firmware versions with more features. As things scale, I think you'd get administratively worn out first (quite a large Dashboard page with hundreds of rules) before the APs ran out of horsepower. So yes, there's certainly additional processing happening if you've got several hundred firewall and traffic shaping rules, but in most cases when there's that many (I've only seen a few) the question is "why" and there's typically a much better way to consolidate rules down to a more manageable and reasonable level. If you're on the scale of a few dozen rules at most (very common), I'd say the impact is minimal to negligible in most use cases.
