I'm not trying to argue Sobers was a great bowler, but as a batsman who averages high 50s, as a fifth bowling option he is very good.
And here's a few thoughts on that...
His average and SR are fairly poor, I'd agree. But they do compare with others who are considered decent bowlers, such as Warwick Armstrong, Trevor Hohns, Ken McKay, Dan Vettori and Tim May. So really, he is a batsman who averages high 50s, while also being comparable to some other bowlers who were selected on bowling alone.
Just as a couple of examples, here are some test records that show the role Sobers had to play in the WI's team (stock bowler bowling a lot of overs)...
1st Test: England v West Indies at Manchester, Jun 2-4, 1966 | Cricket Scorecard | ESPN Cricinfo
4th Test: West Indies v Australia at Bridgetown, May 5-11, 1965 | Cricket Scorecard | ESPN Cricinfo
3rd Test: West Indies v India at Georgetown, Mar 19-24, 1971 | Cricket Scorecard | ESPN Cricinfo
His role probably wasn't to be a strike bowler, but to bowl a heap of economical overs with Hall/Griffith/Gibbs bowled the rest.
Just as a side note, check out the last test there. Sobers bowled 43 overs (3 wickets), then came out and made 108* with the bat. Oh Shane Watson, why can't you?