The field and default is consistent with the numbering and behavior of the same named field in Microsoft Project and has been like this for a very long time.
I'm not aware of plans to change the current default.
For Projects, Portfolio Optimization app lets you calculate project priorities based on KPIs of your choosing and our upcoming Capacity Planning capability lets you fulfil resourcing requests according to that Priority.
500 just means an item hasn't been prioritized.You can change the default number using a workflow rule or just save 0 value into a standard template.
If it's not useful at all, you can remove the field from all Profiles.