It's illegal. I love all this justification from people who get their hand caught in the cookie jar, but it's illegal.
The problem is that the only way to tell that it becomes a problem for another bowler is when the other bowler trips and busts his a-double-s and then it's too late.
If I see someone using it, I politely ask them to stop and I tell them why. I'll offer my shoe brushes and some advice on how to condition their soles.
What happens at that point depends on their attitude. I've only reported one person to a league official before; she was a older female bowler in a mixed league who would hit her sole so much she'd form an EZ-slide cloud in the pit. One of her teammates fell following her on the lanes once and was out for a month, but she was the team captain so it wasn't like they could throw her off the team.
We were bowling on her pair, and my wife has a very iffy back. The first time I saw the cloud go up, I politely asked her not to do that. She responded by telling me she'd been doing it for 20 years and wasn't going to stop doing it that night. I said, "Yes ma'am, you are," and went straight to the league president.
She stared at my team for the rest of the night like she was going to kill us, but what are you going to do? She hardly walked fast enough to slide, anyway.
And, it's against the rules. I really don't need anything more than that.
Jess