Are you thinking of always keeping the chain on one of the front sprockets? That would be like converting your 2×5 =10 speed into a 1×5 = 5 speed. If yes, then there is one disadvantage I can think of. Bike chain is made to bend in one direction – around the sprockets. It has very little capability for lateral movement. Ideally you should use rear sprockets #1, #2, #3 when your front sprocket is on #1 and rear sprockets #3, #4,# 5 when you are on front sprocket #2. That minimizes lateral chain loading. I’m guessing you are not an Olympic athlete nor is your bike built right at the stress limits. So you are ok with only one front sprocket. Just be aware of the lateral loading and try not to spend all your time at the extremes.