With the ability to only use one slot you would miss out on half of the channels. You might be able to use local and not international or vice versa.
Actually, radios that can't handle dual timeslots won't talk to repeaters at all. They can only talk to other single-timeslot radios in simplex mode, and waste about 50% of the battery power when transmitting. The DMR digital modulation and voice encoding scheme only requires the transmitter to run 50% of the time--33 milliseconds on, and then 33 milliseconds off. This allows 2 radios to transmit on the same frequency; one transmits for 33 milliseconds, then it shuts off and the other radio transmits for 33 milliseconds. The MD-380 and MD-390 work this way, so they can talk to repeaters, and they use about 50% of the battery power in digital mode they do in analog FM mode.
The MD-398 is a stupid design; it transmits continuously. Instead of shutting down after 33 milliseconds and waiting 33 milliseconds before transmitting again, it rebroadcasts the first 33 milliseconds' data in the second 33 millisecond time slot, which wastes battery power and guarantees repeater incompatibility because the handheld has to stop transmitting in order to sync with the repeater and receive signaling, status, and control data from the repeater. The Baofeng DM-5R does the same thing, so I wouldn't recommend either radio.
If you get the MD-380 or MD-390, you get a radio that complies with DMR standards and does everything except trunking, and uses battery power much more efficiently.
Also, going over 5-7 watts with a handheld is unwise due to RF exposure.