I would unflinchingly do without 25% 'additional functionality' for a 25% uptick in HCM solution usability any day of the week.
This position relates to HR-ERP's, but is even more applicable to niche Talent Management Solutions (e.g., LMS, Compensation Management, Recruiting, Performance Management, Succession Planning) ... because the latter class of solutions are geared for a broader and more diverse user community. Also, since HR-ERP's are leveraged mostly as data repositories/systems of record and administrative processing engines, usability expectations have historically been quite modest.
Presumably, any HR/HCM solution on the market for years is commercially viable -- often measured by whether core business requirements are accounted for within a fully tested product. Commercial viability of an HCM enterprise solution is rarely based on a usability (or even utilization) metric, even though low marks on usability will always undermine broad-based solution adoption, ROI achieved, and the leveraging of that extra functionality you are likely paying for.