Hey Brian:
For reference, here is the type of strike I was mentioning:
I do not necessarily agree with the advice of that locksmith. If a door has previously been specified to have a mortise type lockset, it generally has been deemed as at least a 'security sensitive' opening. Mortise locksets are typically costly, and they are not the 'default door prep' for light-duty openings. In contrast, cylindrical locksets can be purchased quite inexpensively and typically are used on less vulnerable doors.
Fundamentally, having a separate deadbolt and latch is seen as a 'higher security' solution (ie: two points of securing the door, vs a single point) In this way, mortise locksets are 'more secure' than cylindrical locksets.
In most cases, pulling a mortise set and swapping it out with a cylindrical leverset can be considered a 'step down' in hardware. Granted, this is pure opinion on my part, but I think many door hardware types hold the same opinion. While a mortise-prepped door can be adapted for a cylindrical lock quite easily, it still may require a variety of door wrap plates and strike blanks like you mention.
The locksmith you spoke with may have been able to use a less expensive strike, but he ultimately recommended you buy/sell new hardware, strike blanks, and more labor time installing everything than if attempting to use the mortise lock already in place.