I found some time ago myself thinking that 50-135 is "not long enough and thereby useless for this price" etc etc
Now when I got this thing absolutely unplanned (was a one-time-opportunity to get it on such terms), I must say one thing - quantity can not be any kind of replacement for quality - I
had Sigma 50-150 in mind, but now I'm not.
Longer lens of some kind is still in future plans, but now it's more probable to have a 12-24 as next target...
70-200 and 50-135 are quite different things and should not be considered as replacements to each other - 50-135 is lightweight enough to be a walkaround lens for... say... street photography, but 70-200 will be more suited for shooting bigger events - probably using monopod or tripod when needed.
P.S. Now I understand, why 50-135 has no tripod mount - it is just lighter enough than Tokina 535...