Ziggys_Friend once bubbled...
Since PSI is almost universally used in the USA, unless you're going to be primarily diving in metric communities you really need to stay PSI. This is true for a number of reasons.
1) Most likely YOU think PSI and during an underwater emergency you want things as simple as possible.
2) Lots of decisions (dive duration, distance, turn around point, Rock Bottom Time etc.) are related to your tank pressure and all those decisions must be linked. For example, if you're having your tank filled to 3000PSI and you know in an emergency it is going to take you and your buddy 12 minutes to get safely to the surface, you're going to have to calculate what that means in tank pressure. i.e. the dive is over when you reach X PSI (say 1000). Moreover, you need to be able to figure that number for each dive and with different tanks. Adding a conversion from PSI to Bar each time is just too much work.
3a) Your SPG is one of your team's resources and your buddy(s) need to be able to read it too. Most of us "speak" PSI and don't know BAR. If I check with you to make sure you're ok and you tell me how much pressure you have in BAR's, it's going to make no sense and just confuse me. Even more importantly, if we're in a crisis and I'm OOA and you're sharing with me, we both want me to be able to read your pressure gauge! (Believe me, in an OOA situation, being able to show your buddy you have plenty of air for the two of us is - important.)
3b) As part of your teams resources, you may NEED me to be able to read your SPG easily. For example, just this morning I had an OOA emergency drill where my buddy lost his mask underwater and couldn't see. I then went OOA and needed him to share his air with me. No problem, once he realized my need (by touch since he couldn't see) we started sharing air. However, at this point neither of us knew how much air he had and until we knew that, we couldn't decide how to proceed. I therefore had to read his SPG and make that determination. If it had been in BARs I wouldn't have had a clue.
~<//><
P.S. Good news - we lived to dive another day [not that it was pretty].