I carry a handheld GPS in my g-suit pocket whenever the wx is marginal at the home drome, or whenever I'm out-n-back or XC.
First of all, there are some nasty IFR EPs in the T-38 (right gen fail w/no crossover, TR failure, etc...), and I like having some peace of mind. Secondly, I consider the GPS as a form of insurance from getting violated (sts) when Stanley can't set-up the Navaids correctly or is dorking-up the fix-to-fix. (Studs have a wonderful, magic quality of being total SA black holes...myself as a stud included.) Third, a handheld GPS has some magic flight planning and fuel planning software that can glonckulate fuel faster/more accurately than my 386-proccessor of a brain can.
It's a worthwhile investment as far as I'm concerned...especially if your aircraft is limited to vintage-1950s navaids. Professionals take pride in their equipment: Police Officers are picky about the weapons/protective gear they use to keep them safe, Professional Chefs have to use quality knives/tools to ensure their food is delicious, etc...
...why should we, as pilots, be any different?
Just one Lt's opinion...