The frequency of these events is quite acceptable at the moment, and I expected problems when I first lowered the downstream target SNRM from the standard 6 dB to 3 (this is ADSL2). In any case, if a line goes down momentarily the other two lines are still functional, so I probably don't notice.
It's only going to be a possible problem with certain applications: How much of a practical disruptive effect it might have depends on how fast the routers can switch over and whether something like Netflix can cope with a short outage on one line - don't know what happens about retransmission with Netflix’s service. Clearly TCP won't care, retransmission will simply cover up such an event. Anyway, because I have several lines, the short interruptions may not be really disturbing me. If I start to use VoIP properly, this could be very bad news though.